linux-kernel - Re: xgetbv nondeterminism

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Date:   Thu, 15 Jun 2017 15:40:42 -0700
From:   "H.J. Lu" <hjl.tools@...il.com>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     Dave Hansen <dave.hansen@...el.com>,
        "Robert O'Callahan" <robert@...llahan.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        X86 ML <x86@...nel.org>
Subject: Re: xgetbv nondeterminism

On Thu, Jun 15, 2017 at 3:18 PM, Andy Lutomirski <luto@...nel.org> wrote:
> On Thu, Jun 15, 2017 at 7:33 AM, Dave Hansen <dave.hansen@...el.com> wrote:
>> On 06/14/2017 10:18 PM, Andy Lutomirski wrote:
>>> Dave, why is XINUSE exposed at all to userspace?
>>
>> You need it for XSAVEOPT when it is using the init optimization to be
>> able to tell which state was written and which state in the XSAVE buffer
>> is potentially stale with respect to what's in the registers.  I guess
>> you can just use XSAVE instead of XSAVEOPT, though.
>>
>> As you pointed out, if you are using XSAVEC's compaction features by
>> leaving bits unset in the requested feature bitmap registers, you have
>> no idea how much data XSAVEC will write, unless you read XINUSE with
>> XGETBV.  But, you can get around *that* by just presizing the XSAVE
>> buffer to be big.
>
> I imagine that, if you're going to save, do something quick, and
> restore, you'd be better off allocating a big buffer rather than
> trying to find the smallest buffer you can get away with by reading
> XINUSE.  Also, what happens if XINUSE nondeterministically changes out
> from under you before you do XSAVEC?  I assume you can avoid this
> becoming a problem by using RFBM carefully.
>
>>
>> So, I guess that leaves its use to just figuring out how much XSAVEOPT
>> (and friends) are going to write.
>>
>>> To be fair, glibc uses this new XGETBV feature, but I suspect its
>>> usage is rather dubious.  Shouldn't it just do XSAVEC directly rather
>>> than rolling its own code?
>>
>> A quick grep through my glibc source only shows XGETBV(0) used which
>> reads XCR0.  I don't see any XGETBV(1) which reads XINUSE.  Did I miss it.
>
> Take a look at sysdeps/x86_64/dl-trampoline.h in a new enough version.

I wrote a test to compare latency against different approaches.   This
is on Skylake:

[hjl@...-skl-1 glibc-test]$ make
./test
move    : 47212
fxsave  : 719440
xsave   : 925146
xsavec  : 811036
xsave_state_size: 1088
xsave_state_comp_size: 896

load/store is about 17X faster than xsavec.

I put my hjl/pr21265/xsavec branch at

https://sourceware.org/git/?p=glibc.git;a=summary

It uses xsave/xsave/xsavec in _dl_runtime_resolve.

-- 
H.J.

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