Messages in this thread | | | From | Andy Lutomirski <> | Date | Thu, 15 Jun 2017 15:45:32 -0700 | Subject | Re: xgetbv nondeterminism |
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On Thu, Jun 15, 2017 at 3:40 PM, H.J. Lu <hjl.tools@gmail.com> wrote: > On Thu, Jun 15, 2017 at 3:18 PM, Andy Lutomirski <luto@kernel.org> wrote: >> On Thu, Jun 15, 2017 at 7:33 AM, Dave Hansen <dave.hansen@intel.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@gnu-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?
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