Messages in this thread | | | Date | Tue, 7 Apr 2020 14:33:48 +0200 | From | Peter Zijlstra <> | Subject | Re: [RFC PATCH v2] x86/arch_prctl: Add ARCH_SET_XCR0 to set XCR0 per-thread |
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On Mon, Apr 06, 2020 at 09:53:40PM -0700, Kyle Huey wrote: > On Mon, Apr 6, 2020 at 9:45 PM Keno Fischer <keno@juliacomputing.com> wrote: > > > > On Mon, Apr 6, 2020 at 11:58 PM Andy Lutomirski <luto@amacapital.net> wrote: > > > > > > > > > > On Apr 6, 2020, at 6:13 PM, Keno Fischer <keno@juliacomputing.com> wrote: > > > > > > > > This is a follow-up to my from two-years ago [1]. > > > > > > Your changelog is missing an explanation of why this is useful. Why would a user program want to change XCR0? > > > > Ah, sorry - I wasn't sure what the convention was around repeating the > > applicable parts from the v1 changelog in this email. > > Here's the description from the v1 patch: > > > > > The rr (http://rr-project.org/) debugger provides user space > > > record-and-replay functionality by carefully controlling the process > > > environment in order to ensure completely deterministic execution > > > of recorded traces. The recently added ARCH_SET_CPUID arch_prctl > > > allows rr to move traces across (Intel) machines, by allowing cpuid > > > invocations to be reliably recorded and replayed. This works very > > > well, with one catch: It is currently not possible to replay a > > > recording from a machine supporting a smaller set of XCR0 state > > > components on one supporting a larger set. This is because the > > > value of XCR0 is observable in userspace (either by explicit > > > xgetbv or by looking at the result of xsave) and since glibc > > > does observe this value, replay divergence is almost immediate. > > > I also suspect that people interested in process (or container) > > > live-migration may eventually care about this if a migration happens > > > in between a userspace xsave and a corresponding xrstor. > > > > > > We encounter this problem quite frequently since most of our users > > > are using pre-Skylake systems (and thus don't support the AVX512 > > > state components), while we recently upgraded our main development > > > machines to Skylake. > > > > Basically, for rr to work, we need to tightly control any user-visible > > CPU behavior, > > either by putting in the CPU in the right state or by trapping and emulating > > (as we do for rdtsc, cpuid, etc). XCR0 controls a bunch of > > user-visible CPU behavior, > > namely: > > 1) The size of the xsave region if xsave is passed an all-ones mask > > (which is fairly common) > > 2) The return value of xgetbv > > It's mentioned elsewhere, but I want to emphasize that the return > value of xgetbv is the big one because the dynamic linker uses this. > rr trace portability is essentially limited to machines with identical > xcr0 values because of it.
I'm thinking just exposing that value is doable in a much less objectionable fashion, no?
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