Messages in this thread | | | Date | Tue, 20 Mar 2018 11:54:27 +0100 | From | Ingo Molnar <> | Subject | Re: [RFC PATCH 0/3] kernel: add support for 256-bit IO access |
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* Thomas Gleixner <tglx@linutronix.de> wrote:
> On Tue, 20 Mar 2018, Ingo Molnar wrote: > > * Thomas Gleixner <tglx@linutronix.de> wrote: > > > > > > So I do think we could do more in this area to improve driver performance, if the > > > > code is correct and if there's actual benchmarks that are showing real benefits. > > > > > > If it's about hotpath performance I'm all for it, but the use case here is > > > a debug facility... > > > > > > And if we go down that road then we want a AVX based memcpy() > > > implementation which is runtime conditional on the feature bit(s) and > > > length dependent. Just slapping a readqq() at it and use it in a loop does > > > not make any sense. > > > > Yeah, so generic memcpy() replacement is only feasible I think if the most > > optimistic implementation is actually correct: > > > > - if no preempt disable()/enable() is required > > > > - if direct access to the AVX[2] registers does not disturb legacy FPU state in > > any fashion > > > > - if direct access to the AVX[2] registers cannot raise weird exceptions or have > > weird behavior if the FPU control word is modified to non-standard values by > > untrusted user-space > > > > If we have to touch the FPU tag or control words then it's probably only good for > > a specialized API. > > I did not mean to have a general memcpy replacement. Rather something like > magic_memcpy() which falls back to memcpy when AVX is not usable or the > length does not justify the AVX stuff at all.
OK, fair enough.
Note that a generic version might still be worth trying out, if and only if it's safe to access those vector registers directly: modern x86 CPUs will do their non-constant memcpy()s via the common memcpy_erms() function - which could in theory be an easy common point to be (cpufeatures-) patched to an AVX2 variant, if size (and alignment, perhaps) is a multiple of 32 bytes or so.
Assuming it's correct with arbitrary user-space FPU state and if it results in any measurable speedups, which might not be the case: ERMS is supposed to be very fast.
So even if it's possible (which it might not be), it could end up being slower than the ERMS version.
Thanks,
Ingo
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