linux-kernel - Re: xgetbv nondeterminism

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Message-ID: <CALCETrXdV_T-o+336FAhn7aqZDGgLnxrSz6NomQP21P1BSCeGg@mail.gmail.com>
Date:   Thu, 15 Jun 2017 16:28:26 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     "H.J. Lu" <hjl.tools@...il.com>
Cc:     Andy Lutomirski <luto@...nel.org>,
        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 4:11 PM, H.J. Lu <hjl.tools@...il.com> wrote:
> 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.
>

Maybe I'm just being dense, but why?  What does ld.so need to do to
resolve a symbol and update the GOT that requires using extended
state?