Messages in this thread | | | From | Andy Lutomirski <> | Date | Mon, 29 Feb 2016 12:45:58 -0800 | Subject | Re: [PATCH 02/10] x86/entry/compat: In SYSENTER, sink AC clearing below the existing FLAGS test |
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On Mon, Feb 29, 2016 at 12:39 PM, Borislav Petkov <bp@alien8.de> wrote: > On Sun, Feb 28, 2016 at 09:28:47PM -0800, Andy Lutomirski wrote: >> 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..7c8e72da7654 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 $X86_EFLAGS_NT|X86_EFLAGS_AC, EFLAGS(%rsp) >> jnz .Lsysenter_fix_flags >> .Lsysenter_flags_fixed: > > Do I see it correctly that with this change, that .Lsysenter_fix_flags: > is going to be visited each time on SMAP machines and then we can get > rid of it? The reason for it was not to pollute I$ as the comment says > but that happening now anyway... >
I don't think so. Sensible user programs shouldn't set AC in the first place.
--Andy
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