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| Started by | Andy Lutomirski <luto@kernel.org> |
|---|---|
| First post | 2016-03-10 04:10 +0100 |
| Last post | 2016-03-10 12:10 +0100 |
| Articles | 2 — 2 participants |
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[PATCH v2 02/12] x86/entry/compat: In SYSENTER, sink AC clearing below the existing FLAGS test Andy Lutomirski <luto@kernel.org> - 2016-03-10 04:10 +0100
[tip:x86/asm] x86/entry/compat: In SYSENTER, sink AC clearing below the existing FLAGS test tip-bot for Andy Lutomirski <tipbot@zytor.com> - 2016-03-10 12:10 +0100
| From | Andy Lutomirski <luto@kernel.org> |
|---|---|
| Date | 2016-03-10 04:10 +0100 |
| Subject | [PATCH v2 02/12] x86/entry/compat: In SYSENTER, sink AC clearing below the existing FLAGS test |
| Message-ID | <raZ97-HP-5@gated-at.bofh.it> |
CLAC is slow, and the SYSENTER code already has an unlikely path that runs if unusual flags are set. Drop the CLAC and instead rely on the unlikely path to clear AC. This seems to save ~24 cycles on my Skylake laptop. (Hey, Intel, make this faster please!) Signed-off-by: Andy Lutomirski <luto@kernel.org> --- arch/x86/entry/entry_64_compat.S | 8 +++----- 1 file changed, 3 insertions(+), 5 deletions(-) diff --git a/arch/x86/entry/entry_64_compat.S b/arch/x86/entry/entry_64_compat.S index 89bcb4979e7a..8d3728131809 100644 --- a/arch/x86/entry/entry_64_compat.S +++ b/arch/x86/entry/entry_64_compat.S @@ -66,8 +66,6 @@ ENTRY(entry_SYSENTER_compat) */ pushfq /* pt_regs->flags (except IF = 0) */ orl $X86_EFLAGS_IF, (%rsp) /* Fix saved flags */ - ASM_CLAC /* Clear AC after saving FLAGS */ - pushq $__USER32_CS /* pt_regs->cs */ xorq %r8,%r8 pushq %r8 /* pt_regs->ip = 0 (placeholder) */ @@ -90,9 +88,9 @@ ENTRY(entry_SYSENTER_compat) cld /* - * Sysenter doesn't filter flags, so we need to clear NT + * SYSENTER doesn't filter flags, so we need to clear NT and AC * ourselves. To save a few cycles, we can check whether - * NT was set instead of doing an unconditional popfq. + * either was set instead of doing an unconditional popfq. * This needs to happen before enabling interrupts so that * we don't get preempted with NT set. * @@ -102,7 +100,7 @@ ENTRY(entry_SYSENTER_compat) * we're keeping that code behind a branch which will predict as * not-taken and therefore its instructions won't be fetched. */ - testl $X86_EFLAGS_NT, EFLAGS(%rsp) + testl $X86_EFLAGS_NT|X86_EFLAGS_AC, EFLAGS(%rsp) jnz .Lsysenter_fix_flags .Lsysenter_flags_fixed: -- 2.5.0
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| From | tip-bot for Andy Lutomirski <tipbot@zytor.com> |
|---|---|
| Date | 2016-03-10 12:10 +0100 |
| Subject | [tip:x86/asm] x86/entry/compat: In SYSENTER, sink AC clearing below the existing FLAGS test |
| Message-ID | <rb6DF-6cT-43@gated-at.bofh.it> |
| In reply to | #1354800 |
Commit-ID: e786041153df6343169373177248abfab5c5ac1b Gitweb: http://git.kernel.org/tip/e786041153df6343169373177248abfab5c5ac1b Author: Andy Lutomirski <luto@kernel.org> AuthorDate: Wed, 9 Mar 2016 19:00:25 -0800 Committer: Ingo Molnar <mingo@kernel.org> CommitDate: Thu, 10 Mar 2016 09:48:12 +0100 x86/entry/compat: In SYSENTER, sink AC clearing below the existing FLAGS test CLAC is slow, and the SYSENTER code already has an unlikely path that runs if unusual flags are set. Drop the CLAC and instead rely on the unlikely path to clear AC. This seems to save ~24 cycles on my Skylake laptop. (Hey, Intel, make this faster please!) Signed-off-by: Andy Lutomirski <luto@kernel.org> Cc: Andrew Cooper <andrew.cooper3@citrix.com> Cc: Andy Lutomirski <luto@amacapital.net> Cc: Borislav Petkov <bp@alien8.de> Cc: Brian Gerst <brgerst@gmail.com> Cc: Denys Vlasenko <dvlasenk@redhat.com> Cc: H. Peter Anvin <hpa@zytor.com> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Oleg Nesterov <oleg@redhat.com> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Thomas Gleixner <tglx@linutronix.de> Link: http://lkml.kernel.org/r/90d6db2189f9add83bc7bddd75a0c19ebbd676b2.1457578375.git.luto@kernel.org Signed-off-by: Ingo Molnar <mingo@kernel.org> --- arch/x86/entry/entry_64_compat.S | 8 +++----- 1 file changed, 3 insertions(+), 5 deletions(-) diff --git a/arch/x86/entry/entry_64_compat.S b/arch/x86/entry/entry_64_compat.S index 3c990ee..da5c194 100644 --- a/arch/x86/entry/entry_64_compat.S +++ b/arch/x86/entry/entry_64_compat.S @@ -66,8 +66,6 @@ ENTRY(entry_SYSENTER_compat) */ pushfq /* pt_regs->flags (except IF = 0) */ orl $X86_EFLAGS_IF, (%rsp) /* Fix saved flags */ - ASM_CLAC /* Clear AC after saving FLAGS */ - pushq $__USER32_CS /* pt_regs->cs */ xorq %r8,%r8 pushq %r8 /* pt_regs->ip = 0 (placeholder) */ @@ -90,9 +88,9 @@ ENTRY(entry_SYSENTER_compat) cld /* - * Sysenter doesn't filter flags, so we need to clear NT + * SYSENTER doesn't filter flags, so we need to clear NT and AC * ourselves. To save a few cycles, we can check whether - * NT was set instead of doing an unconditional popfq. + * either was set instead of doing an unconditional popfq. * This needs to happen before enabling interrupts so that * we don't get preempted with NT set. * @@ -102,7 +100,7 @@ ENTRY(entry_SYSENTER_compat) * we're keeping that code behind a branch which will predict as * not-taken and therefore its instructions won't be fetched. */ - testl $X86_EFLAGS_NT, EFLAGS(%rsp) + testl $X86_EFLAGS_NT|X86_EFLAGS_AC, EFLAGS(%rsp) jnz .Lsysenter_fix_flags .Lsysenter_flags_fixed:
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