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Groups > linux.kernel > #1351055
| From | Andy Lutomirski <luto@kernel.org> |
|---|---|
| Newsgroups | linux.kernel |
| Subject | [PATCH v2 02/10] x86/entry/compat: In SYSENTER, sink AC clearing below the existing FLAGS test |
| Date | 2016-03-06 07:00 +0100 |
| Message-ID | <r9zTt-Q8-21@gated-at.bofh.it> (permalink) |
| References | <r9zTr-Q8-3@gated-at.bofh.it> |
| Organization | linux.* mail to news gateway |
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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[PATCH 00/10] x86: Various SYSENTER/SYSEXIT/#DB fixes and cleanups Andy Lutomirski <luto@kernel.org> - 2016-03-06 07:00 +0100
[PATCH v2 06/10] x86/traps: Clear DR6 early in do_debug and improve the comment Andy Lutomirski <luto@kernel.org> - 2016-03-06 07:00 +0100
[PATCH v2 01/10] selftests/x86: In syscall_nt, test NT|TF as well Andy Lutomirski <luto@kernel.org> - 2016-03-06 07:00 +0100
[PATCH v2 05/10] x86/traps: Clear TIF_BLOCKSTEP on all debug exceptions Andy Lutomirski <luto@kernel.org> - 2016-03-06 07:00 +0100
[PATCH v2 03/10] x86/entry/32: Filter NT and speed up AC filtering in SYSENTER Andy Lutomirski <luto@kernel.org> - 2016-03-06 07:00 +0100
[PATCH v2 02/10] x86/entry/compat: In SYSENTER, sink AC clearing below the existing FLAGS test Andy Lutomirski <luto@kernel.org> - 2016-03-06 07:00 +0100
Re: [PATCH 00/10] x86: Various SYSENTER/SYSEXIT/#DB fixes and cleanups Ingo Molnar <mingo@kernel.org> - 2016-03-06 09:30 +0100
Re: [PATCH 00/10] x86: Various SYSENTER/SYSEXIT/#DB fixes and cleanups Andy Lutomirski <luto@amacapital.net> - 2016-03-06 17:20 +0100
Re: [PATCH 00/10] x86: Various SYSENTER/SYSEXIT/#DB fixes and cleanups Ingo Molnar <mingo@kernel.org> - 2016-03-07 09:30 +0100
Re: [PATCH 00/10] x86: Various SYSENTER/SYSEXIT/#DB fixes and cleanups Ingo Molnar <mingo@kernel.org> - 2016-03-06 09:40 +0100
Re: [PATCH 00/10] x86: Various SYSENTER/SYSEXIT/#DB fixes and cleanups Andy Lutomirski <luto@amacapital.net> - 2016-03-06 17:20 +0100
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