Messages in this thread | | | From | Andy Lutomirski <> | Date | Thu, 15 Jun 2017 16:28:26 -0700 | Subject | Re: xgetbv nondeterminism |
| |
On Thu, Jun 15, 2017 at 4:11 PM, H.J. Lu <hjl.tools@gmail.com> wrote: > On Thu, Jun 15, 2017 at 3:45 PM, Andy Lutomirski <luto@kernel.org> wrote: >> 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? > > 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?
| |