GNU Linux-libre 5.15.54-gnu

[releases.git] / arch / x86 / entry / entry_64.S

/* SPDX-License-Identifier: GPL-2.0 */
/*
 *  linux/arch/x86_64/entry.S
 *
 *  Copyright (C) 1991, 1992  Linus Torvalds
 *  Copyright (C) 2000, 2001, 2002  Andi Kleen SuSE Labs
 *  Copyright (C) 2000  Pavel Machek <pavel@suse.cz>
 *
 * entry.S contains the system-call and fault low-level handling routines.
 *
 * Some of this is documented in Documentation/x86/entry_64.rst
 *
 * A note on terminology:
 * - iret frame:        Architecture defined interrupt frame from SS to RIP
 *                      at the top of the kernel process stack.
 *
 * Some macro usage:
 * - SYM_FUNC_START/END:Define functions in the symbol table.
 * - idtentry:          Define exception entry points.
 */
#include <linux/linkage.h>
#include <asm/segment.h>
#include <asm/cache.h>
#include <asm/errno.h>
#include <asm/asm-offsets.h>
#include <asm/msr.h>
#include <asm/unistd.h>
#include <asm/thread_info.h>
#include <asm/hw_irq.h>
#include <asm/page_types.h>
#include <asm/irqflags.h>
#include <asm/paravirt.h>
#include <asm/percpu.h>
#include <asm/asm.h>
#include <asm/smap.h>
#include <asm/pgtable_types.h>
#include <asm/export.h>
#include <asm/frame.h>
#include <asm/trapnr.h>
#include <asm/nospec-branch.h>
#include <asm/fsgsbase.h>
#include <linux/err.h>

#include "calling.h"

.code64
.section .entry.text, "ax"

/*
 * 64-bit SYSCALL instruction entry. Up to 6 arguments in registers.
 *
 * This is the only entry point used for 64-bit system calls.  The
 * hardware interface is reasonably well designed and the register to
 * argument mapping Linux uses fits well with the registers that are
 * available when SYSCALL is used.
 *
 * SYSCALL instructions can be found inlined in libc implementations as
 * well as some other programs and libraries.  There are also a handful
 * of SYSCALL instructions in the vDSO used, for example, as a
 * clock_gettimeofday fallback.
 *
 * 64-bit SYSCALL saves rip to rcx, clears rflags.RF, then saves rflags to r11,
 * then loads new ss, cs, and rip from previously programmed MSRs.
 * rflags gets masked by a value from another MSR (so CLD and CLAC
 * are not needed). SYSCALL does not save anything on the stack
 * and does not change rsp.
 *
 * Registers on entry:
 * rax  system call number
 * rcx  return address
 * r11  saved rflags (note: r11 is callee-clobbered register in C ABI)
 * rdi  arg0
 * rsi  arg1
 * rdx  arg2
 * r10  arg3 (needs to be moved to rcx to conform to C ABI)
 * r8   arg4
 * r9   arg5
 * (note: r12-r15, rbp, rbx are callee-preserved in C ABI)
 *
 * Only called from user space.
 *
 * When user can change pt_regs->foo always force IRET. That is because
 * it deals with uncanonical addresses better. SYSRET has trouble
 * with them due to bugs in both AMD and Intel CPUs.
 */

SYM_CODE_START(entry_SYSCALL_64)
        UNWIND_HINT_EMPTY

        swapgs
        /* tss.sp2 is scratch space. */
        movq    %rsp, PER_CPU_VAR(cpu_tss_rw + TSS_sp2)
        SWITCH_TO_KERNEL_CR3 scratch_reg=%rsp
        movq    PER_CPU_VAR(cpu_current_top_of_stack), %rsp

SYM_INNER_LABEL(entry_SYSCALL_64_safe_stack, SYM_L_GLOBAL)

        /* Construct struct pt_regs on stack */
        pushq   $__USER_DS                              /* pt_regs->ss */
        pushq   PER_CPU_VAR(cpu_tss_rw + TSS_sp2)       /* pt_regs->sp */
        pushq   %r11                                    /* pt_regs->flags */
        pushq   $__USER_CS                              /* pt_regs->cs */
        pushq   %rcx                                    /* pt_regs->ip */
SYM_INNER_LABEL(entry_SYSCALL_64_after_hwframe, SYM_L_GLOBAL)
        pushq   %rax                                    /* pt_regs->orig_ax */

        PUSH_AND_CLEAR_REGS rax=$-ENOSYS

        /* IRQs are off. */
        movq    %rsp, %rdi
        /* Sign extend the lower 32bit as syscall numbers are treated as int */
        movslq  %eax, %rsi
        call    do_syscall_64           /* returns with IRQs disabled */

        /*
         * Try to use SYSRET instead of IRET if we're returning to
         * a completely clean 64-bit userspace context.  If we're not,
         * go to the slow exit path.
         * In the Xen PV case we must use iret anyway.
         */

        ALTERNATIVE "", "jmp    swapgs_restore_regs_and_return_to_usermode", \
                X86_FEATURE_XENPV

        movq    RCX(%rsp), %rcx
        movq    RIP(%rsp), %r11

        cmpq    %rcx, %r11      /* SYSRET requires RCX == RIP */
        jne     swapgs_restore_regs_and_return_to_usermode

        /*
         * On Intel CPUs, SYSRET with non-canonical RCX/RIP will #GP
         * in kernel space.  This essentially lets the user take over
         * the kernel, since userspace controls RSP.
         *
         * If width of "canonical tail" ever becomes variable, this will need
         * to be updated to remain correct on both old and new CPUs.
         *
         * Change top bits to match most significant bit (47th or 56th bit
         * depending on paging mode) in the address.
         */
#ifdef CONFIG_X86_5LEVEL
        ALTERNATIVE "shl $(64 - 48), %rcx; sar $(64 - 48), %rcx", \
                "shl $(64 - 57), %rcx; sar $(64 - 57), %rcx", X86_FEATURE_LA57
#else
        shl     $(64 - (__VIRTUAL_MASK_SHIFT+1)), %rcx
        sar     $(64 - (__VIRTUAL_MASK_SHIFT+1)), %rcx
#endif

        /* If this changed %rcx, it was not canonical */
        cmpq    %rcx, %r11
        jne     swapgs_restore_regs_and_return_to_usermode

        cmpq    $__USER_CS, CS(%rsp)            /* CS must match SYSRET */
        jne     swapgs_restore_regs_and_return_to_usermode

        movq    R11(%rsp), %r11
        cmpq    %r11, EFLAGS(%rsp)              /* R11 == RFLAGS */
        jne     swapgs_restore_regs_and_return_to_usermode

        /*
         * SYSCALL clears RF when it saves RFLAGS in R11 and SYSRET cannot
         * restore RF properly. If the slowpath sets it for whatever reason, we
         * need to restore it correctly.
         *
         * SYSRET can restore TF, but unlike IRET, restoring TF results in a
         * trap from userspace immediately after SYSRET.  This would cause an
         * infinite loop whenever #DB happens with register state that satisfies
         * the opportunistic SYSRET conditions.  For example, single-stepping
         * this user code:
         *
         *           movq       $stuck_here, %rcx
         *           pushfq
         *           popq %r11
         *   stuck_here:
         *
         * would never get past 'stuck_here'.
         */
        testq   $(X86_EFLAGS_RF|X86_EFLAGS_TF), %r11
        jnz     swapgs_restore_regs_and_return_to_usermode

        /* nothing to check for RSP */

        cmpq    $__USER_DS, SS(%rsp)            /* SS must match SYSRET */
        jne     swapgs_restore_regs_and_return_to_usermode

        /*
         * We win! This label is here just for ease of understanding
         * perf profiles. Nothing jumps here.
         */
syscall_return_via_sysret:
        /* rcx and r11 are already restored (see code above) */
        POP_REGS pop_rdi=0 skip_r11rcx=1

        /*
         * Now all regs are restored except RSP and RDI.
         * Save old stack pointer and switch to trampoline stack.
         */
        movq    %rsp, %rdi
        movq    PER_CPU_VAR(cpu_tss_rw + TSS_sp0), %rsp
        UNWIND_HINT_EMPTY

        pushq   RSP-RDI(%rdi)   /* RSP */
        pushq   (%rdi)          /* RDI */

        /*
         * We are on the trampoline stack.  All regs except RDI are live.
         * We can do future final exit work right here.
         */
        STACKLEAK_ERASE_NOCLOBBER

        SWITCH_TO_USER_CR3_STACK scratch_reg=%rdi

        popq    %rdi
        popq    %rsp
        swapgs
        sysretq
SYM_CODE_END(entry_SYSCALL_64)

/*
 * %rdi: prev task
 * %rsi: next task
 */
.pushsection .text, "ax"
SYM_FUNC_START(__switch_to_asm)
        /*
         * Save callee-saved registers
         * This must match the order in inactive_task_frame
         */
        pushq   %rbp
        pushq   %rbx
        pushq   %r12
        pushq   %r13
        pushq   %r14
        pushq   %r15

        /* switch stack */
        movq    %rsp, TASK_threadsp(%rdi)
        movq    TASK_threadsp(%rsi), %rsp

#ifdef CONFIG_STACKPROTECTOR
        movq    TASK_stack_canary(%rsi), %rbx
        movq    %rbx, PER_CPU_VAR(fixed_percpu_data) + stack_canary_offset
#endif

#ifdef CONFIG_RETPOLINE
        /*
         * When switching from a shallower to a deeper call stack
         * the RSB may either underflow or use entries populated
         * with userspace addresses. On CPUs where those concerns
         * exist, overwrite the RSB with entries which capture
         * speculative execution to prevent attack.
         */
        FILL_RETURN_BUFFER %r12, RSB_CLEAR_LOOPS, X86_FEATURE_RSB_CTXSW
#endif

        /* restore callee-saved registers */
        popq    %r15
        popq    %r14
        popq    %r13
        popq    %r12
        popq    %rbx
        popq    %rbp

        jmp     __switch_to
SYM_FUNC_END(__switch_to_asm)
.popsection

/*
 * A newly forked process directly context switches into this address.
 *
 * rax: prev task we switched from
 * rbx: kernel thread func (NULL for user thread)
 * r12: kernel thread arg
 */
.pushsection .text, "ax"
SYM_CODE_START(ret_from_fork)
        UNWIND_HINT_EMPTY
        movq    %rax, %rdi
        call    schedule_tail                   /* rdi: 'prev' task parameter */

        testq   %rbx, %rbx                      /* from kernel_thread? */
        jnz     1f                              /* kernel threads are uncommon */

2:
        UNWIND_HINT_REGS
        movq    %rsp, %rdi
        call    syscall_exit_to_user_mode       /* returns with IRQs disabled */
        jmp     swapgs_restore_regs_and_return_to_usermode

1:
        /* kernel thread */
        UNWIND_HINT_EMPTY
        movq    %r12, %rdi
        CALL_NOSPEC rbx
        /*
         * A kernel thread is allowed to return here after successfully
         * calling kernel_execve().  Exit to userspace to complete the execve()
         * syscall.
         */
        movq    $0, RAX(%rsp)
        jmp     2b
SYM_CODE_END(ret_from_fork)
.popsection

.macro DEBUG_ENTRY_ASSERT_IRQS_OFF
#ifdef CONFIG_DEBUG_ENTRY
        pushq %rax
        SAVE_FLAGS
        testl $X86_EFLAGS_IF, %eax
        jz .Lokay_\@
        ud2
.Lokay_\@:
        popq %rax
#endif
.endm

/**
 * idtentry_body - Macro to emit code calling the C function
 * @cfunc:              C function to be called
 * @has_error_code:     Hardware pushed error code on stack
 */
.macro idtentry_body cfunc has_error_code:req

        call    error_entry
        UNWIND_HINT_REGS

        movq    %rsp, %rdi                      /* pt_regs pointer into 1st argument*/

        .if \has_error_code == 1
                movq    ORIG_RAX(%rsp), %rsi    /* get error code into 2nd argument*/
                movq    $-1, ORIG_RAX(%rsp)     /* no syscall to restart */
        .endif

        call    \cfunc

        jmp     error_return
.endm

/**
 * idtentry - Macro to generate entry stubs for simple IDT entries
 * @vector:             Vector number
 * @asmsym:             ASM symbol for the entry point
 * @cfunc:              C function to be called
 * @has_error_code:     Hardware pushed error code on stack
 *
 * The macro emits code to set up the kernel context for straight forward
 * and simple IDT entries. No IST stack, no paranoid entry checks.
 */
.macro idtentry vector asmsym cfunc has_error_code:req
SYM_CODE_START(\asmsym)
        UNWIND_HINT_IRET_REGS offset=\has_error_code*8
        ASM_CLAC

        .if \has_error_code == 0
                pushq   $-1                     /* ORIG_RAX: no syscall to restart */
        .endif

        .if \vector == X86_TRAP_BP
                /*
                 * If coming from kernel space, create a 6-word gap to allow the
                 * int3 handler to emulate a call instruction.
                 */
                testb   $3, CS-ORIG_RAX(%rsp)
                jnz     .Lfrom_usermode_no_gap_\@
                .rept   6
                pushq   5*8(%rsp)
                .endr
                UNWIND_HINT_IRET_REGS offset=8
.Lfrom_usermode_no_gap_\@:
        .endif

        idtentry_body \cfunc \has_error_code

_ASM_NOKPROBE(\asmsym)
SYM_CODE_END(\asmsym)
.endm

/*
 * Interrupt entry/exit.
 *
 + The interrupt stubs push (vector) onto the stack, which is the error_code
 * position of idtentry exceptions, and jump to one of the two idtentry points
 * (common/spurious).
 *
 * common_interrupt is a hotpath, align it to a cache line
 */
.macro idtentry_irq vector cfunc
        .p2align CONFIG_X86_L1_CACHE_SHIFT
        idtentry \vector asm_\cfunc \cfunc has_error_code=1
.endm

/*
 * System vectors which invoke their handlers directly and are not
 * going through the regular common device interrupt handling code.
 */
.macro idtentry_sysvec vector cfunc
        idtentry \vector asm_\cfunc \cfunc has_error_code=0
.endm

/**
 * idtentry_mce_db - Macro to generate entry stubs for #MC and #DB
 * @vector:             Vector number
 * @asmsym:             ASM symbol for the entry point
 * @cfunc:              C function to be called
 *
 * The macro emits code to set up the kernel context for #MC and #DB
 *
 * If the entry comes from user space it uses the normal entry path
 * including the return to user space work and preemption checks on
 * exit.
 *
 * If hits in kernel mode then it needs to go through the paranoid
 * entry as the exception can hit any random state. No preemption
 * check on exit to keep the paranoid path simple.
 */
.macro idtentry_mce_db vector asmsym cfunc
SYM_CODE_START(\asmsym)
        UNWIND_HINT_IRET_REGS
        ASM_CLAC

        pushq   $-1                     /* ORIG_RAX: no syscall to restart */

        /*
         * If the entry is from userspace, switch stacks and treat it as
         * a normal entry.
         */
        testb   $3, CS-ORIG_RAX(%rsp)
        jnz     .Lfrom_usermode_switch_stack_\@

        /* paranoid_entry returns GS information for paranoid_exit in EBX. */
        call    paranoid_entry

        UNWIND_HINT_REGS

        movq    %rsp, %rdi              /* pt_regs pointer */

        call    \cfunc

        jmp     paranoid_exit

        /* Switch to the regular task stack and use the noist entry point */
.Lfrom_usermode_switch_stack_\@:
        idtentry_body noist_\cfunc, has_error_code=0

_ASM_NOKPROBE(\asmsym)
SYM_CODE_END(\asmsym)
.endm

#ifdef CONFIG_AMD_MEM_ENCRYPT
/**
 * idtentry_vc - Macro to generate entry stub for #VC
 * @vector:             Vector number
 * @asmsym:             ASM symbol for the entry point
 * @cfunc:              C function to be called
 *
 * The macro emits code to set up the kernel context for #VC. The #VC handler
 * runs on an IST stack and needs to be able to cause nested #VC exceptions.
 *
 * To make this work the #VC entry code tries its best to pretend it doesn't use
 * an IST stack by switching to the task stack if coming from user-space (which
 * includes early SYSCALL entry path) or back to the stack in the IRET frame if
 * entered from kernel-mode.
 *
 * If entered from kernel-mode the return stack is validated first, and if it is
 * not safe to use (e.g. because it points to the entry stack) the #VC handler
 * will switch to a fall-back stack (VC2) and call a special handler function.
 *
 * The macro is only used for one vector, but it is planned to be extended in
 * the future for the #HV exception.
 */
.macro idtentry_vc vector asmsym cfunc
SYM_CODE_START(\asmsym)
        UNWIND_HINT_IRET_REGS
        ASM_CLAC

        /*
         * If the entry is from userspace, switch stacks and treat it as
         * a normal entry.
         */
        testb   $3, CS-ORIG_RAX(%rsp)
        jnz     .Lfrom_usermode_switch_stack_\@

        /*
         * paranoid_entry returns SWAPGS flag for paranoid_exit in EBX.
         * EBX == 0 -> SWAPGS, EBX == 1 -> no SWAPGS
         */
        call    paranoid_entry

        UNWIND_HINT_REGS

        /*
         * Switch off the IST stack to make it free for nested exceptions. The
         * vc_switch_off_ist() function will switch back to the interrupted
         * stack if it is safe to do so. If not it switches to the VC fall-back
         * stack.
         */
        movq    %rsp, %rdi              /* pt_regs pointer */
        call    vc_switch_off_ist
        movq    %rax, %rsp              /* Switch to new stack */

        ENCODE_FRAME_POINTER
        UNWIND_HINT_REGS

        /* Update pt_regs */
        movq    ORIG_RAX(%rsp), %rsi    /* get error code into 2nd argument*/
        movq    $-1, ORIG_RAX(%rsp)     /* no syscall to restart */

        movq    %rsp, %rdi              /* pt_regs pointer */

        call    kernel_\cfunc

        /*
         * No need to switch back to the IST stack. The current stack is either
         * identical to the stack in the IRET frame or the VC fall-back stack,
         * so it is definitely mapped even with PTI enabled.
         */
        jmp     paranoid_exit

        /* Switch to the regular task stack */
.Lfrom_usermode_switch_stack_\@:
        idtentry_body user_\cfunc, has_error_code=1

_ASM_NOKPROBE(\asmsym)
SYM_CODE_END(\asmsym)
.endm
#endif

/*
 * Double fault entry. Straight paranoid. No checks from which context
 * this comes because for the espfix induced #DF this would do the wrong
 * thing.
 */
.macro idtentry_df vector asmsym cfunc
SYM_CODE_START(\asmsym)
        UNWIND_HINT_IRET_REGS offset=8
        ASM_CLAC

        /* paranoid_entry returns GS information for paranoid_exit in EBX. */
        call    paranoid_entry
        UNWIND_HINT_REGS

        movq    %rsp, %rdi              /* pt_regs pointer into first argument */
        movq    ORIG_RAX(%rsp), %rsi    /* get error code into 2nd argument*/
        movq    $-1, ORIG_RAX(%rsp)     /* no syscall to restart */
        call    \cfunc

        jmp     paranoid_exit

_ASM_NOKPROBE(\asmsym)
SYM_CODE_END(\asmsym)
.endm

/*
 * Include the defines which emit the idt entries which are shared
 * shared between 32 and 64 bit and emit the __irqentry_text_* markers
 * so the stacktrace boundary checks work.
 */
        .align 16
        .globl __irqentry_text_start
__irqentry_text_start:

#include <asm/idtentry.h>

        .align 16
        .globl __irqentry_text_end
__irqentry_text_end:

SYM_CODE_START_LOCAL(common_interrupt_return)
SYM_INNER_LABEL(swapgs_restore_regs_and_return_to_usermode, SYM_L_GLOBAL)
#ifdef CONFIG_DEBUG_ENTRY
        /* Assert that pt_regs indicates user mode. */
        testb   $3, CS(%rsp)
        jnz     1f
        ud2
1:
#endif
#ifdef CONFIG_XEN_PV
        ALTERNATIVE "", "jmp xenpv_restore_regs_and_return_to_usermode", X86_FEATURE_XENPV
#endif

        POP_REGS pop_rdi=0

        /*
         * The stack is now user RDI, orig_ax, RIP, CS, EFLAGS, RSP, SS.
         * Save old stack pointer and switch to trampoline stack.
         */
        movq    %rsp, %rdi
        movq    PER_CPU_VAR(cpu_tss_rw + TSS_sp0), %rsp
        UNWIND_HINT_EMPTY

        /* Copy the IRET frame to the trampoline stack. */
        pushq   6*8(%rdi)       /* SS */
        pushq   5*8(%rdi)       /* RSP */
        pushq   4*8(%rdi)       /* EFLAGS */
        pushq   3*8(%rdi)       /* CS */
        pushq   2*8(%rdi)       /* RIP */

        /* Push user RDI on the trampoline stack. */
        pushq   (%rdi)

        /*
         * We are on the trampoline stack.  All regs except RDI are live.
         * We can do future final exit work right here.
         */
        STACKLEAK_ERASE_NOCLOBBER

        SWITCH_TO_USER_CR3_STACK scratch_reg=%rdi

        /* Restore RDI. */
        popq    %rdi
        SWAPGS
        INTERRUPT_RETURN


SYM_INNER_LABEL(restore_regs_and_return_to_kernel, SYM_L_GLOBAL)
#ifdef CONFIG_DEBUG_ENTRY
        /* Assert that pt_regs indicates kernel mode. */
        testb   $3, CS(%rsp)
        jz      1f
        ud2
1:
#endif
        POP_REGS
        addq    $8, %rsp        /* skip regs->orig_ax */
        /*
         * ARCH_HAS_MEMBARRIER_SYNC_CORE rely on IRET core serialization
         * when returning from IPI handler.
         */
        INTERRUPT_RETURN

SYM_INNER_LABEL_ALIGN(native_iret, SYM_L_GLOBAL)
        UNWIND_HINT_IRET_REGS
        /*
         * Are we returning to a stack segment from the LDT?  Note: in
         * 64-bit mode SS:RSP on the exception stack is always valid.
         */
#ifdef CONFIG_X86_ESPFIX64
        testb   $4, (SS-RIP)(%rsp)
        jnz     native_irq_return_ldt
#endif

SYM_INNER_LABEL(native_irq_return_iret, SYM_L_GLOBAL)
        /*
         * This may fault.  Non-paranoid faults on return to userspace are
         * handled by fixup_bad_iret.  These include #SS, #GP, and #NP.
         * Double-faults due to espfix64 are handled in exc_double_fault.
         * Other faults here are fatal.
         */
        iretq

#ifdef CONFIG_X86_ESPFIX64
native_irq_return_ldt:
        /*
         * We are running with user GSBASE.  All GPRs contain their user
         * values.  We have a percpu ESPFIX stack that is eight slots
         * long (see ESPFIX_STACK_SIZE).  espfix_waddr points to the bottom
         * of the ESPFIX stack.
         *
         * We clobber RAX and RDI in this code.  We stash RDI on the
         * normal stack and RAX on the ESPFIX stack.
         *
         * The ESPFIX stack layout we set up looks like this:
         *
         * --- top of ESPFIX stack ---
         * SS
         * RSP
         * RFLAGS
         * CS
         * RIP  <-- RSP points here when we're done
         * RAX  <-- espfix_waddr points here
         * --- bottom of ESPFIX stack ---
         */

        pushq   %rdi                            /* Stash user RDI */
        swapgs                                  /* to kernel GS */
        SWITCH_TO_KERNEL_CR3 scratch_reg=%rdi   /* to kernel CR3 */

        movq    PER_CPU_VAR(espfix_waddr), %rdi
        movq    %rax, (0*8)(%rdi)               /* user RAX */
        movq    (1*8)(%rsp), %rax               /* user RIP */
        movq    %rax, (1*8)(%rdi)
        movq    (2*8)(%rsp), %rax               /* user CS */
        movq    %rax, (2*8)(%rdi)
        movq    (3*8)(%rsp), %rax               /* user RFLAGS */
        movq    %rax, (3*8)(%rdi)
        movq    (5*8)(%rsp), %rax               /* user SS */
        movq    %rax, (5*8)(%rdi)
        movq    (4*8)(%rsp), %rax               /* user RSP */
        movq    %rax, (4*8)(%rdi)
        /* Now RAX == RSP. */

        andl    $0xffff0000, %eax               /* RAX = (RSP & 0xffff0000) */

        /*
         * espfix_stack[31:16] == 0.  The page tables are set up such that
         * (espfix_stack | (X & 0xffff0000)) points to a read-only alias of
         * espfix_waddr for any X.  That is, there are 65536 RO aliases of
         * the same page.  Set up RSP so that RSP[31:16] contains the
         * respective 16 bits of the /userspace/ RSP and RSP nonetheless
         * still points to an RO alias of the ESPFIX stack.
         */
        orq     PER_CPU_VAR(espfix_stack), %rax

        SWITCH_TO_USER_CR3_STACK scratch_reg=%rdi
        swapgs                                  /* to user GS */
        popq    %rdi                            /* Restore user RDI */

        movq    %rax, %rsp
        UNWIND_HINT_IRET_REGS offset=8

        /*
         * At this point, we cannot write to the stack any more, but we can
         * still read.
         */
        popq    %rax                            /* Restore user RAX */

        /*
         * RSP now points to an ordinary IRET frame, except that the page
         * is read-only and RSP[31:16] are preloaded with the userspace
         * values.  We can now IRET back to userspace.
         */
        jmp     native_irq_return_iret
#endif
SYM_CODE_END(common_interrupt_return)
_ASM_NOKPROBE(common_interrupt_return)

/*
 * Reload gs selector with exception handling
 * edi:  new selector
 *
 * Is in entry.text as it shouldn't be instrumented.
 */
SYM_FUNC_START(asm_load_gs_index)
        FRAME_BEGIN
        swapgs
.Lgs_change:
        movl    %edi, %gs
2:      ALTERNATIVE "", "mfence", X86_BUG_SWAPGS_FENCE
        swapgs
        FRAME_END
        RET
SYM_FUNC_END(asm_load_gs_index)
EXPORT_SYMBOL(asm_load_gs_index)

        _ASM_EXTABLE(.Lgs_change, .Lbad_gs)
        .section .fixup, "ax"
        /* running with kernelgs */
SYM_CODE_START_LOCAL_NOALIGN(.Lbad_gs)
        swapgs                                  /* switch back to user gs */
.macro ZAP_GS
        /* This can't be a string because the preprocessor needs to see it. */
        movl $__USER_DS, %eax
        movl %eax, %gs
.endm
        ALTERNATIVE "", "ZAP_GS", X86_BUG_NULL_SEG
        xorl    %eax, %eax
        movl    %eax, %gs
        jmp     2b
SYM_CODE_END(.Lbad_gs)
        .previous

#ifdef CONFIG_XEN_PV
/*
 * A note on the "critical region" in our callback handler.
 * We want to avoid stacking callback handlers due to events occurring
 * during handling of the last event. To do this, we keep events disabled
 * until we've done all processing. HOWEVER, we must enable events before
 * popping the stack frame (can't be done atomically) and so it would still
 * be possible to get enough handler activations to overflow the stack.
 * Although unlikely, bugs of that kind are hard to track down, so we'd
 * like to avoid the possibility.
 * So, on entry to the handler we detect whether we interrupted an
 * existing activation in its critical region -- if so, we pop the current
 * activation and restart the handler using the previous one.
 *
 * C calling convention: exc_xen_hypervisor_callback(struct *pt_regs)
 */
SYM_CODE_START_LOCAL(exc_xen_hypervisor_callback)

/*
 * Since we don't modify %rdi, evtchn_do_upall(struct *pt_regs) will
 * see the correct pointer to the pt_regs
 */
        UNWIND_HINT_FUNC
        movq    %rdi, %rsp                      /* we don't return, adjust the stack frame */
        UNWIND_HINT_REGS

        call    xen_pv_evtchn_do_upcall

        jmp     error_return
SYM_CODE_END(exc_xen_hypervisor_callback)

/*
 * Hypervisor uses this for application faults while it executes.
 * We get here for two reasons:
 *  1. Fault while reloading DS, ES, FS or GS
 *  2. Fault while executing IRET
 * Category 1 we do not need to fix up as Xen has already reloaded all segment
 * registers that could be reloaded and zeroed the others.
 * Category 2 we fix up by killing the current process. We cannot use the
 * normal Linux return path in this case because if we use the IRET hypercall
 * to pop the stack frame we end up in an infinite loop of failsafe callbacks.
 * We distinguish between categories by comparing each saved segment register
 * with its current contents: any discrepancy means we in category 1.
 */
SYM_CODE_START(xen_failsafe_callback)
        UNWIND_HINT_EMPTY
        movl    %ds, %ecx
        cmpw    %cx, 0x10(%rsp)
        jne     1f
        movl    %es, %ecx
        cmpw    %cx, 0x18(%rsp)
        jne     1f
        movl    %fs, %ecx
        cmpw    %cx, 0x20(%rsp)
        jne     1f
        movl    %gs, %ecx
        cmpw    %cx, 0x28(%rsp)
        jne     1f
        /* All segments match their saved values => Category 2 (Bad IRET). */
        movq    (%rsp), %rcx
        movq    8(%rsp), %r11
        addq    $0x30, %rsp
        pushq   $0                              /* RIP */
        UNWIND_HINT_IRET_REGS offset=8
        jmp     asm_exc_general_protection
1:      /* Segment mismatch => Category 1 (Bad segment). Retry the IRET. */
        movq    (%rsp), %rcx
        movq    8(%rsp), %r11
        addq    $0x30, %rsp
        UNWIND_HINT_IRET_REGS
        pushq   $-1 /* orig_ax = -1 => not a system call */
        PUSH_AND_CLEAR_REGS
        ENCODE_FRAME_POINTER
        jmp     error_return
SYM_CODE_END(xen_failsafe_callback)
#endif /* CONFIG_XEN_PV */

/*
 * Save all registers in pt_regs. Return GSBASE related information
 * in EBX depending on the availability of the FSGSBASE instructions:
 *
 * FSGSBASE     R/EBX
 *     N        0 -> SWAPGS on exit
 *              1 -> no SWAPGS on exit
 *
 *     Y        GSBASE value at entry, must be restored in paranoid_exit
 */
SYM_CODE_START_LOCAL(paranoid_entry)
        UNWIND_HINT_FUNC
        cld
        PUSH_AND_CLEAR_REGS save_ret=1
        ENCODE_FRAME_POINTER 8

        /*
         * Always stash CR3 in %r14.  This value will be restored,
         * verbatim, at exit.  Needed if paranoid_entry interrupted
         * another entry that already switched to the user CR3 value
         * but has not yet returned to userspace.
         *
         * This is also why CS (stashed in the "iret frame" by the
         * hardware at entry) can not be used: this may be a return
         * to kernel code, but with a user CR3 value.
         *
         * Switching CR3 does not depend on kernel GSBASE so it can
         * be done before switching to the kernel GSBASE. This is
         * required for FSGSBASE because the kernel GSBASE has to
         * be retrieved from a kernel internal table.
         */
        SAVE_AND_SWITCH_TO_KERNEL_CR3 scratch_reg=%rax save_reg=%r14

        /*
         * Handling GSBASE depends on the availability of FSGSBASE.
         *
         * Without FSGSBASE the kernel enforces that negative GSBASE
         * values indicate kernel GSBASE. With FSGSBASE no assumptions
         * can be made about the GSBASE value when entering from user
         * space.
         */
        ALTERNATIVE "jmp .Lparanoid_entry_checkgs", "", X86_FEATURE_FSGSBASE

        /*
         * Read the current GSBASE and store it in %rbx unconditionally,
         * retrieve and set the current CPUs kernel GSBASE. The stored value
         * has to be restored in paranoid_exit unconditionally.
         *
         * The unconditional write to GS base below ensures that no subsequent
         * loads based on a mispredicted GS base can happen, therefore no LFENCE
         * is needed here.
         */
        SAVE_AND_SET_GSBASE scratch_reg=%rax save_reg=%rbx
        RET

.Lparanoid_entry_checkgs:
        /* EBX = 1 -> kernel GSBASE active, no restore required */
        movl    $1, %ebx

        /*
         * The kernel-enforced convention is a negative GSBASE indicates
         * a kernel value. No SWAPGS needed on entry and exit.
         */
        movl    $MSR_GS_BASE, %ecx
        rdmsr
        testl   %edx, %edx
        js      .Lparanoid_kernel_gsbase

        /* EBX = 0 -> SWAPGS required on exit */
        xorl    %ebx, %ebx
        swapgs
.Lparanoid_kernel_gsbase:

        FENCE_SWAPGS_KERNEL_ENTRY
        RET
SYM_CODE_END(paranoid_entry)

/*
 * "Paranoid" exit path from exception stack.  This is invoked
 * only on return from non-NMI IST interrupts that came
 * from kernel space.
 *
 * We may be returning to very strange contexts (e.g. very early
 * in syscall entry), so checking for preemption here would
 * be complicated.  Fortunately, there's no good reason to try
 * to handle preemption here.
 *
 * R/EBX contains the GSBASE related information depending on the
 * availability of the FSGSBASE instructions:
 *
 * FSGSBASE     R/EBX
 *     N        0 -> SWAPGS on exit
 *              1 -> no SWAPGS on exit
 *
 *     Y        User space GSBASE, must be restored unconditionally
 */
SYM_CODE_START_LOCAL(paranoid_exit)
        UNWIND_HINT_REGS
        /*
         * The order of operations is important. RESTORE_CR3 requires
         * kernel GSBASE.
         *
         * NB to anyone to try to optimize this code: this code does
         * not execute at all for exceptions from user mode. Those
         * exceptions go through error_exit instead.
         */
        RESTORE_CR3     scratch_reg=%rax save_reg=%r14

        /* Handle the three GSBASE cases */
        ALTERNATIVE "jmp .Lparanoid_exit_checkgs", "", X86_FEATURE_FSGSBASE

        /* With FSGSBASE enabled, unconditionally restore GSBASE */
        wrgsbase        %rbx
        jmp             restore_regs_and_return_to_kernel

.Lparanoid_exit_checkgs:
        /* On non-FSGSBASE systems, conditionally do SWAPGS */
        testl           %ebx, %ebx
        jnz             restore_regs_and_return_to_kernel

        /* We are returning to a context with user GSBASE */
        swapgs
        jmp             restore_regs_and_return_to_kernel
SYM_CODE_END(paranoid_exit)

/*
 * Save all registers in pt_regs, and switch GS if needed.
 */
SYM_CODE_START_LOCAL(error_entry)
        UNWIND_HINT_FUNC
        cld
        PUSH_AND_CLEAR_REGS save_ret=1
        ENCODE_FRAME_POINTER 8
        testb   $3, CS+8(%rsp)
        jz      .Lerror_kernelspace

        /*
         * We entered from user mode or we're pretending to have entered
         * from user mode due to an IRET fault.
         */
        SWAPGS
        FENCE_SWAPGS_USER_ENTRY
        /* We have user CR3.  Change to kernel CR3. */
        SWITCH_TO_KERNEL_CR3 scratch_reg=%rax

.Lerror_entry_from_usermode_after_swapgs:
        /* Put us onto the real thread stack. */
        popq    %r12                            /* save return addr in %12 */
        movq    %rsp, %rdi                      /* arg0 = pt_regs pointer */
        call    sync_regs
        movq    %rax, %rsp                      /* switch stack */
        ENCODE_FRAME_POINTER
        pushq   %r12
        RET

        /*
         * There are two places in the kernel that can potentially fault with
         * usergs. Handle them here.  B stepping K8s sometimes report a
         * truncated RIP for IRET exceptions returning to compat mode. Check
         * for these here too.
         */
.Lerror_kernelspace:
        leaq    native_irq_return_iret(%rip), %rcx
        cmpq    %rcx, RIP+8(%rsp)
        je      .Lerror_bad_iret
        movl    %ecx, %eax                      /* zero extend */
        cmpq    %rax, RIP+8(%rsp)
        je      .Lbstep_iret
        cmpq    $.Lgs_change, RIP+8(%rsp)
        jne     .Lerror_entry_done_lfence

        /*
         * hack: .Lgs_change can fail with user gsbase.  If this happens, fix up
         * gsbase and proceed.  We'll fix up the exception and land in
         * .Lgs_change's error handler with kernel gsbase.
         */
        SWAPGS

        /*
         * Issue an LFENCE to prevent GS speculation, regardless of whether it is a
         * kernel or user gsbase.
         */
.Lerror_entry_done_lfence:
        FENCE_SWAPGS_KERNEL_ENTRY
        RET

.Lbstep_iret:
        /* Fix truncated RIP */
        movq    %rcx, RIP+8(%rsp)
        /* fall through */

.Lerror_bad_iret:
        /*
         * We came from an IRET to user mode, so we have user
         * gsbase and CR3.  Switch to kernel gsbase and CR3:
         */
        SWAPGS
        FENCE_SWAPGS_USER_ENTRY
        SWITCH_TO_KERNEL_CR3 scratch_reg=%rax

        /*
         * Pretend that the exception came from user mode: set up pt_regs
         * as if we faulted immediately after IRET.
         */
        mov     %rsp, %rdi
        call    fixup_bad_iret
        mov     %rax, %rsp
        jmp     .Lerror_entry_from_usermode_after_swapgs
SYM_CODE_END(error_entry)

SYM_CODE_START_LOCAL(error_return)
        UNWIND_HINT_REGS
        DEBUG_ENTRY_ASSERT_IRQS_OFF
        testb   $3, CS(%rsp)
        jz      restore_regs_and_return_to_kernel
        jmp     swapgs_restore_regs_and_return_to_usermode
SYM_CODE_END(error_return)

/*
 * Runs on exception stack.  Xen PV does not go through this path at all,
 * so we can use real assembly here.
 *
 * Registers:
 *      %r14: Used to save/restore the CR3 of the interrupted context
 *            when PAGE_TABLE_ISOLATION is in use.  Do not clobber.
 */
SYM_CODE_START(asm_exc_nmi)
        UNWIND_HINT_IRET_REGS

        /*
         * We allow breakpoints in NMIs. If a breakpoint occurs, then
         * the iretq it performs will take us out of NMI context.
         * This means that we can have nested NMIs where the next
         * NMI is using the top of the stack of the previous NMI. We
         * can't let it execute because the nested NMI will corrupt the
         * stack of the previous NMI. NMI handlers are not re-entrant
         * anyway.
         *
         * To handle this case we do the following:
         *  Check the a special location on the stack that contains
         *  a variable that is set when NMIs are executing.
         *  The interrupted task's stack is also checked to see if it
         *  is an NMI stack.
         *  If the variable is not set and the stack is not the NMI
         *  stack then:
         *    o Set the special variable on the stack
         *    o Copy the interrupt frame into an "outermost" location on the
         *      stack
         *    o Copy the interrupt frame into an "iret" location on the stack
         *    o Continue processing the NMI
         *  If the variable is set or the previous stack is the NMI stack:
         *    o Modify the "iret" location to jump to the repeat_nmi
         *    o return back to the first NMI
         *
         * Now on exit of the first NMI, we first clear the stack variable
         * The NMI stack will tell any nested NMIs at that point that it is
         * nested. Then we pop the stack normally with iret, and if there was
         * a nested NMI that updated the copy interrupt stack frame, a
         * jump will be made to the repeat_nmi code that will handle the second
         * NMI.
         *
         * However, espfix prevents us from directly returning to userspace
         * with a single IRET instruction.  Similarly, IRET to user mode
         * can fault.  We therefore handle NMIs from user space like
         * other IST entries.
         */

        ASM_CLAC

        /* Use %rdx as our temp variable throughout */
        pushq   %rdx

        testb   $3, CS-RIP+8(%rsp)
        jz      .Lnmi_from_kernel

        /*
         * NMI from user mode.  We need to run on the thread stack, but we
         * can't go through the normal entry paths: NMIs are masked, and
         * we don't want to enable interrupts, because then we'll end
         * up in an awkward situation in which IRQs are on but NMIs
         * are off.
         *
         * We also must not push anything to the stack before switching
         * stacks lest we corrupt the "NMI executing" variable.
         */

        swapgs
        cld
        FENCE_SWAPGS_USER_ENTRY
        SWITCH_TO_KERNEL_CR3 scratch_reg=%rdx
        movq    %rsp, %rdx
        movq    PER_CPU_VAR(cpu_current_top_of_stack), %rsp
        UNWIND_HINT_IRET_REGS base=%rdx offset=8
        pushq   5*8(%rdx)       /* pt_regs->ss */
        pushq   4*8(%rdx)       /* pt_regs->rsp */
        pushq   3*8(%rdx)       /* pt_regs->flags */
        pushq   2*8(%rdx)       /* pt_regs->cs */
        pushq   1*8(%rdx)       /* pt_regs->rip */
        UNWIND_HINT_IRET_REGS
        pushq   $-1             /* pt_regs->orig_ax */
        PUSH_AND_CLEAR_REGS rdx=(%rdx)
        ENCODE_FRAME_POINTER

        /*
         * At this point we no longer need to worry about stack damage
         * due to nesting -- we're on the normal thread stack and we're
         * done with the NMI stack.
         */

        movq    %rsp, %rdi
        movq    $-1, %rsi
        call    exc_nmi

        /*
         * Return back to user mode.  We must *not* do the normal exit
         * work, because we don't want to enable interrupts.
         */
        jmp     swapgs_restore_regs_and_return_to_usermode

.Lnmi_from_kernel:
        /*
         * Here's what our stack frame will look like:
         * +---------------------------------------------------------+
         * | original SS                                             |
         * | original Return RSP                                     |
         * | original RFLAGS                                         |
         * | original CS                                             |
         * | original RIP                                            |
         * +---------------------------------------------------------+
         * | temp storage for rdx                                    |
         * +---------------------------------------------------------+
         * | "NMI executing" variable                                |
         * +---------------------------------------------------------+
         * | iret SS          } Copied from "outermost" frame        |
         * | iret Return RSP  } on each loop iteration; overwritten  |
         * | iret RFLAGS      } by a nested NMI to force another     |
         * | iret CS          } iteration if needed.                 |
         * | iret RIP         }                                      |
         * +---------------------------------------------------------+
         * | outermost SS          } initialized in first_nmi;       |
         * | outermost Return RSP  } will not be changed before      |
         * | outermost RFLAGS      } NMI processing is done.         |
         * | outermost CS          } Copied to "iret" frame on each  |
         * | outermost RIP         } iteration.                      |
         * +---------------------------------------------------------+
         * | pt_regs                                                 |
         * +---------------------------------------------------------+
         *
         * The "original" frame is used by hardware.  Before re-enabling
         * NMIs, we need to be done with it, and we need to leave enough
         * space for the asm code here.
         *
         * We return by executing IRET while RSP points to the "iret" frame.
         * That will either return for real or it will loop back into NMI
         * processing.
         *
         * The "outermost" frame is copied to the "iret" frame on each
         * iteration of the loop, so each iteration starts with the "iret"
         * frame pointing to the final return target.
         */

        /*
         * Determine whether we're a nested NMI.
         *
         * If we interrupted kernel code between repeat_nmi and
         * end_repeat_nmi, then we are a nested NMI.  We must not
         * modify the "iret" frame because it's being written by
         * the outer NMI.  That's okay; the outer NMI handler is
         * about to about to call exc_nmi() anyway, so we can just
         * resume the outer NMI.
         */

        movq    $repeat_nmi, %rdx
        cmpq    8(%rsp), %rdx
        ja      1f
        movq    $end_repeat_nmi, %rdx
        cmpq    8(%rsp), %rdx
        ja      nested_nmi_out
1:

        /*
         * Now check "NMI executing".  If it's set, then we're nested.
         * This will not detect if we interrupted an outer NMI just
         * before IRET.
         */
        cmpl    $1, -8(%rsp)
        je      nested_nmi

        /*
         * Now test if the previous stack was an NMI stack.  This covers
         * the case where we interrupt an outer NMI after it clears
         * "NMI executing" but before IRET.  We need to be careful, though:
         * there is one case in which RSP could point to the NMI stack
         * despite there being no NMI active: naughty userspace controls
         * RSP at the very beginning of the SYSCALL targets.  We can
         * pull a fast one on naughty userspace, though: we program
         * SYSCALL to mask DF, so userspace cannot cause DF to be set
         * if it controls the kernel's RSP.  We set DF before we clear
         * "NMI executing".
         */
        lea     6*8(%rsp), %rdx
        /* Compare the NMI stack (rdx) with the stack we came from (4*8(%rsp)) */
        cmpq    %rdx, 4*8(%rsp)
        /* If the stack pointer is above the NMI stack, this is a normal NMI */
        ja      first_nmi

        subq    $EXCEPTION_STKSZ, %rdx
        cmpq    %rdx, 4*8(%rsp)
        /* If it is below the NMI stack, it is a normal NMI */
        jb      first_nmi

        /* Ah, it is within the NMI stack. */

        testb   $(X86_EFLAGS_DF >> 8), (3*8 + 1)(%rsp)
        jz      first_nmi       /* RSP was user controlled. */

        /* This is a nested NMI. */

nested_nmi:
        /*
         * Modify the "iret" frame to point to repeat_nmi, forcing another
         * iteration of NMI handling.
         */
        subq    $8, %rsp
        leaq    -10*8(%rsp), %rdx
        pushq   $__KERNEL_DS
        pushq   %rdx
        pushfq
        pushq   $__KERNEL_CS
        pushq   $repeat_nmi

        /* Put stack back */
        addq    $(6*8), %rsp

nested_nmi_out:
        popq    %rdx

        /* We are returning to kernel mode, so this cannot result in a fault. */
        iretq

first_nmi:
        /* Restore rdx. */
        movq    (%rsp), %rdx

        /* Make room for "NMI executing". */
        pushq   $0

        /* Leave room for the "iret" frame */
        subq    $(5*8), %rsp

        /* Copy the "original" frame to the "outermost" frame */
        .rept 5
        pushq   11*8(%rsp)
        .endr
        UNWIND_HINT_IRET_REGS

        /* Everything up to here is safe from nested NMIs */

#ifdef CONFIG_DEBUG_ENTRY
        /*
         * For ease of testing, unmask NMIs right away.  Disabled by
         * default because IRET is very expensive.
         */
        pushq   $0              /* SS */
        pushq   %rsp            /* RSP (minus 8 because of the previous push) */
        addq    $8, (%rsp)      /* Fix up RSP */
        pushfq                  /* RFLAGS */
        pushq   $__KERNEL_CS    /* CS */
        pushq   $1f             /* RIP */
        iretq                   /* continues at repeat_nmi below */
        UNWIND_HINT_IRET_REGS
1:
#endif

repeat_nmi:
        /*
         * If there was a nested NMI, the first NMI's iret will return
         * here. But NMIs are still enabled and we can take another
         * nested NMI. The nested NMI checks the interrupted RIP to see
         * if it is between repeat_nmi and end_repeat_nmi, and if so
         * it will just return, as we are about to repeat an NMI anyway.
         * This makes it safe to copy to the stack frame that a nested
         * NMI will update.
         *
         * RSP is pointing to "outermost RIP".  gsbase is unknown, but, if
         * we're repeating an NMI, gsbase has the same value that it had on
         * the first iteration.  paranoid_entry will load the kernel
         * gsbase if needed before we call exc_nmi().  "NMI executing"
         * is zero.
         */
        movq    $1, 10*8(%rsp)          /* Set "NMI executing". */

        /*
         * Copy the "outermost" frame to the "iret" frame.  NMIs that nest
         * here must not modify the "iret" frame while we're writing to
         * it or it will end up containing garbage.
         */
        addq    $(10*8), %rsp
        .rept 5
        pushq   -6*8(%rsp)
        .endr
        subq    $(5*8), %rsp
end_repeat_nmi:

        /*
         * Everything below this point can be preempted by a nested NMI.
         * If this happens, then the inner NMI will change the "iret"
         * frame to point back to repeat_nmi.
         */
        pushq   $-1                             /* ORIG_RAX: no syscall to restart */

        /*
         * Use paranoid_entry to handle SWAPGS, but no need to use paranoid_exit
         * as we should not be calling schedule in NMI context.
         * Even with normal interrupts enabled. An NMI should not be
         * setting NEED_RESCHED or anything that normal interrupts and
         * exceptions might do.
         */
        call    paranoid_entry
        UNWIND_HINT_REGS

        movq    %rsp, %rdi
        movq    $-1, %rsi
        call    exc_nmi

        /* Always restore stashed CR3 value (see paranoid_entry) */
        RESTORE_CR3 scratch_reg=%r15 save_reg=%r14

        /*
         * The above invocation of paranoid_entry stored the GSBASE
         * related information in R/EBX depending on the availability
         * of FSGSBASE.
         *
         * If FSGSBASE is enabled, restore the saved GSBASE value
         * unconditionally, otherwise take the conditional SWAPGS path.
         */
        ALTERNATIVE "jmp nmi_no_fsgsbase", "", X86_FEATURE_FSGSBASE

        wrgsbase        %rbx
        jmp     nmi_restore

nmi_no_fsgsbase:
        /* EBX == 0 -> invoke SWAPGS */
        testl   %ebx, %ebx
        jnz     nmi_restore

nmi_swapgs:
        swapgs

nmi_restore:
        POP_REGS

        /*
         * Skip orig_ax and the "outermost" frame to point RSP at the "iret"
         * at the "iret" frame.
         */
        addq    $6*8, %rsp

        /*
         * Clear "NMI executing".  Set DF first so that we can easily
         * distinguish the remaining code between here and IRET from
         * the SYSCALL entry and exit paths.
         *
         * We arguably should just inspect RIP instead, but I (Andy) wrote
         * this code when I had the misapprehension that Xen PV supported
         * NMIs, and Xen PV would break that approach.
         */
        std
        movq    $0, 5*8(%rsp)           /* clear "NMI executing" */

        /*
         * iretq reads the "iret" frame and exits the NMI stack in a
         * single instruction.  We are returning to kernel mode, so this
         * cannot result in a fault.  Similarly, we don't need to worry
         * about espfix64 on the way back to kernel mode.
         */
        iretq
SYM_CODE_END(asm_exc_nmi)

#ifndef CONFIG_IA32_EMULATION
/*
 * This handles SYSCALL from 32-bit code.  There is no way to program
 * MSRs to fully disable 32-bit SYSCALL.
 */
SYM_CODE_START(ignore_sysret)
        UNWIND_HINT_EMPTY
        mov     $-ENOSYS, %eax
        sysretl
SYM_CODE_END(ignore_sysret)
#endif

.pushsection .text, "ax"
SYM_CODE_START(rewind_stack_do_exit)
        UNWIND_HINT_FUNC
        /* Prevent any naive code from trying to unwind to our caller. */
        xorl    %ebp, %ebp

        movq    PER_CPU_VAR(cpu_current_top_of_stack), %rax
        leaq    -PTREGS_SIZE(%rax), %rsp
        UNWIND_HINT_REGS

        call    do_exit
SYM_CODE_END(rewind_stack_do_exit)
.popsection