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

Andy Lutomirski
2014-10-01T16:04:27Z

On Wed, Oct 1, 2014 at 8:50 AM, Andy Lutomirski <luto@amacapital.net> wrote:
> 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.)

Agner Fog's tables for Sandy Bridge have 9 uops for popf and
reciprocal throughput 18. sti isn't listed for Sandy Bridge or
anything similar, but cld is 3 uops with reciprocal throughput 4.
Also, popf accesses rsp, and the sysenter code is very heavy on stack
manipulation.

--Andy

>
> 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

--
Andy Lutomirski
AMA Capital Management, LLC