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.
Thanks,
tglx
