Re: [PATCH v2 1/2] x86_64,entry: Filter RFLAGS.NT on entry from userspace

Andy Lutomirski
2014-10-01T15:50:44Z

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