On Thu, Jun 15, 2017 at 4:28 PM, Andy Lutomirski <l...@kernel.org> wrote: > On Thu, Jun 15, 2017 at 4:11 PM, H.J. Lu <hjl.to...@gmail.com> wrote: >> On Thu, Jun 15, 2017 at 3:45 PM, Andy Lutomirski <l...@kernel.org> wrote: >>> On Thu, Jun 15, 2017 at 3:40 PM, H.J. Lu <hjl.to...@gmail.com> wrote: >>>> On Thu, Jun 15, 2017 at 3:18 PM, Andy Lutomirski <l...@kernel.org> wrote: >>>>> On Thu, Jun 15, 2017 at 7:33 AM, Dave Hansen <dave.han...@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? >> >> 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?
Since the first 8 vector registers are used to pass function parameters and ld.so uses vector registers, _dl_runtime_resolve needs to preserve the first 8 vector registers when transferring control to ld.so. -- H.J.
