[Xen-devel] [PATCH v2] x86/xen/64: Rearrange the SYSCALL entries

[Andy Lutomirski <luto@xxxxxxxxxx>]

Xen's raw SYSCALL entries are much less weird than native.  Rather
than fudging them to look like native entries, use the Xen-provided
stack frame directly.

This lets us eliminate entry_SYSCALL_64_after_swapgs and two uses of
the SWAPGS_UNSAFE_STACK paravirt hook.  The SYSENTER code would
benefit from similar treatment.

This makes one change to the native code path: the compat
instruction that clears the high 32 bits of %rax is moved slightly
later.  I'd be surprised if this affects performance at all.

Signed-off-by: Andy Lutomirski <luto@xxxxxxxxxx>
---

Changes from v1 (which I never actually emailed):
 - Fix zero-extension in the compat case.

 arch/x86/entry/entry_64.S        |  9 ++-------
 arch/x86/entry/entry_64_compat.S |  7 +++----
 arch/x86/xen/xen-asm_64.S        | 23 +++++++++--------------
 3 files changed, 14 insertions(+), 25 deletions(-)

diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
index aa58155187c5..7cee92cf807f 100644
--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -142,14 +142,8 @@ ENTRY(entry_SYSCALL_64)
         * We do not frame this tiny irq-off block with TRACE_IRQS_OFF/ON,
         * it is too small to ever cause noticeable irq latency.
         */
-       SWAPGS_UNSAFE_STACK
-       /*
-        * A hypervisor implementation might want to use a label
-        * after the swapgs, so that it can do the swapgs
-        * for the guest and jump here on syscall.
-        */
-GLOBAL(entry_SYSCALL_64_after_swapgs)
 
+       swapgs
        movq    %rsp, PER_CPU_VAR(rsp_scratch)
        movq    PER_CPU_VAR(cpu_current_top_of_stack), %rsp
 
@@ -161,6 +155,7 @@ GLOBAL(entry_SYSCALL_64_after_swapgs)
        pushq   %r11                            /* pt_regs->flags */
        pushq   $__USER_CS                      /* pt_regs->cs */
        pushq   %rcx                            /* pt_regs->ip */
+GLOBAL(entry_SYSCALL_64_after_hwframe)
        pushq   %rax                            /* pt_regs->orig_ax */
        pushq   %rdi                            /* pt_regs->di */
        pushq   %rsi                            /* pt_regs->si */
diff --git a/arch/x86/entry/entry_64_compat.S b/arch/x86/entry/entry_64_compat.S
index e1721dafbcb1..5314d7b8e5ad 100644
--- a/arch/x86/entry/entry_64_compat.S
+++ b/arch/x86/entry/entry_64_compat.S
@@ -183,21 +183,20 @@ ENDPROC(entry_SYSENTER_compat)
  */
 ENTRY(entry_SYSCALL_compat)
        /* Interrupts are off on entry. */
-       SWAPGS_UNSAFE_STACK
+       swapgs
 
        /* Stash user ESP and switch to the kernel stack. */
        movl    %esp, %r8d
        movq    PER_CPU_VAR(cpu_current_top_of_stack), %rsp
 
-       /* Zero-extending 32-bit regs, do not remove */
-       movl    %eax, %eax
-
        /* Construct struct pt_regs on stack */
        pushq   $__USER32_DS            /* pt_regs->ss */
        pushq   %r8                     /* pt_regs->sp */
        pushq   %r11                    /* pt_regs->flags */
        pushq   $__USER32_CS            /* pt_regs->cs */
        pushq   %rcx                    /* pt_regs->ip */
+GLOBAL(entry_SYSCALL_compat_after_hwframe)
+       movl    %eax, %eax              /* discard orig_ax high bits */
        pushq   %rax                    /* pt_regs->orig_ax */
        pushq   %rdi                    /* pt_regs->di */
        pushq   %rsi                    /* pt_regs->si */
diff --git a/arch/x86/xen/xen-asm_64.S b/arch/x86/xen/xen-asm_64.S
index c3df43141e70..a8a4f4c460a6 100644
--- a/arch/x86/xen/xen-asm_64.S
+++ b/arch/x86/xen/xen-asm_64.S
@@ -82,34 +82,29 @@ RELOC(xen_sysret64, 1b+1)
  *     rip
  *     r11
  * rsp->rcx
- *
- * In all the entrypoints, we undo all that to make it look like a
- * CPU-generated syscall/sysenter and jump to the normal entrypoint.
  */
 
-.macro undo_xen_syscall
-       mov 0*8(%rsp), %rcx
-       mov 1*8(%rsp), %r11
-       mov 5*8(%rsp), %rsp
-.endm
-
 /* Normal 64-bit system call target */
 ENTRY(xen_syscall_target)
-       undo_xen_syscall
-       jmp entry_SYSCALL_64_after_swapgs
+       popq %rcx
+       popq %r11
+       jmp entry_SYSCALL_64_after_hwframe
 ENDPROC(xen_syscall_target)
 
 #ifdef CONFIG_IA32_EMULATION
 
 /* 32-bit compat syscall target */
 ENTRY(xen_syscall32_target)
-       undo_xen_syscall
-       jmp entry_SYSCALL_compat
+       popq %rcx
+       popq %r11
+       jmp entry_SYSCALL_compat_after_hwframe
 ENDPROC(xen_syscall32_target)
 
 /* 32-bit compat sysenter target */
 ENTRY(xen_sysenter_target)
-       undo_xen_syscall
+       mov 0*8(%rsp), %rcx
+       mov 1*8(%rsp), %r11
+       mov 5*8(%rsp), %rsp
        jmp entry_SYSENTER_compat
 ENDPROC(xen_sysenter_target)
 
-- 
2.13.3


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