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Subject[PATCH v2] x86/xen/64: Rearrange the SYSCALL entries
Date
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@kernel.org>
---

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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