Messages in this thread | | | From | Andy Lutomirski <> | Date | Wed, 1 Oct 2014 08:50:21 -0700 | Subject | Re: [PATCH v2 1/2] x86_64,entry: Filter RFLAGS.NT on entry from userspace |
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On Oct 1, 2014 8:26 AM, "H. Peter Anvin" <hpa@zytor.com> wrote: > > On 10/01/2014 08:22 AM, H. Peter Anvin wrote: > > On 09/30/2014 09:51 PM, Andy Lutomirski wrote: > >> > >> diff --git a/arch/x86/ia32/ia32entry.S b/arch/x86/ia32/ia32entry.S > >> index 4299eb05023c..44d1dd371454 100644 > >> --- a/arch/x86/ia32/ia32entry.S > >> +++ b/arch/x86/ia32/ia32entry.S > >> @@ -151,6 +151,18 @@ ENTRY(ia32_sysenter_target) > >> 1: movl (%rbp),%ebp > >> _ASM_EXTABLE(1b,ia32_badarg) > >> ASM_CLAC > >> + > >> + /* > >> + * Sysenter doesn't filter flags, so we need to clear NT > >> + * ourselves. To save a few cycles, we can check whether > >> + * NT was set instead of doing an unconditional popfq. > >> + */ > >> + testl $X86_EFLAGS_NT,EFLAGS(%rsp) /* saved EFLAGS match cpu */ > >> + jz 1f > >> + pushq_cfi $(X86_EFLAGS_IF|X86_EFLAGS_FIXED) > >> + popfq_cfi > >> +1: > >> + > > > > I'm wondering if it would be easier to just remove ASM_CLAC and do this > > unconditionally. On SMAP-enabled hardware then that gives us back some > > of the cycles, may make the branch unnecessary. > > > > Heck, we can drop the CLD and the STI as well (with some tweaking in > ia32_badarg.)
I prototyped this, and performance sucked. I suspect that cld and sti are fairly well optimized, that I ended up introducing stalls due to stack manipulation, and that Sandy Bridge's popfq microcode is just not that fast. Maybe I did it wrong. Dunno. Also, I can't benchmark a SMAP machine, since I don't have one. (Does anyone? I'm currently tempted to wait for Skylake before upgrading all my systems.)
In fact, I think we should change all the irqrestore code to do
if (flags & X86_EFLAFS_IF) sti;
I can send a v3 with the unlikely code moved out of line.
--Andy
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