linux-kernel - Re: xgetbv nondeterminism

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Message-ID: <CAMe9rOqrgBD9kafx92q4Khdj9vzAr4bTE2TvBO=TG54ajkseVA@mail.gmail.com>
Date:   Thu, 15 Jun 2017 16:11:29 -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 3:45 PM, Andy Lutomirski <luto@...nel.org> wrote:
> On Thu, Jun 15, 2017 at 3:40 PM, H.J. Lu <hjl.tools@...il.com> wrote:
>> 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.
>
> What is this used for?  Is it just to avoid clobbering argument regs
> when resolving a symbol that uses an ifunc, or is there more to it?

It is used for lazy binding the first time when an external function is called.


-- 
H.J.