* Thomas Gleixner <t...@linutronix.de> wrote:
> On Tue, 20 Mar 2018, Ingo Molnar wrote:
> > * Thomas Gleixner <t...@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
