Re: [PATCH 02/10] x86/entry/compat: In SYSENTER, sink AC clearing below the existing FLAGS test
From: Borislav Petkov
Date: Mon Feb 29 2016 - 15:39:50 EST
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@xxxxxxxxxx>
> 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)
> - * 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 $X86_EFLAGS_NT|X86_EFLAGS_AC, EFLAGS(%rsp)
> jnz .Lsysenter_fix_flags
Do I see it correctly that with this change, that .Lsysenter_fix_flags:
is going to be visited each time on SMAP machines and then we can get
rid of it? The reason for it was not to pollute I$ as the comment says
but that happening now anyway...
ECO tip #101: Trim your mails when you reply.