On Tue, 20 Mar 2018, Ingo Molnar wrote:
> * Thomas Gleixner <t...@linutronix.de> wrote:
>
> > > Useful also for code that needs AVX-like registers to do things like CRCs.
> >
> > x86/crypto/ has a lot of AVX optimized code.
>
> Yeah, that's true, but the crypto code is processing fundamentally bigger
> blocks
> of data, which amortizes the cost of using kernel_fpu_begin()/_end().
Correct.
> So assuming the target driver will only load on modern FPUs I *think* it
> should
> actually be possible to do something like (pseudocode):
>
> vmovdqa %ymm0, 40(%rsp)
> vmovdqa %ymm1, 80(%rsp)
>
> ...
> # use ymm0 and ymm1
> ...
>
> vmovdqa 80(%rsp), %ymm1
> vmovdqa 40(%rsp), %ymm0
>
> ... without using the heavy XSAVE/XRSTOR instructions.
>
> Note that preemption probably still needs to be disabled and possibly there
> are
> other details as well, but there should be no 'heavy' FPU operations.
Emphasis on should :)
> I think this should still preserve all user-space FPU state and shouldn't
> muck up
> any 'weird' user-space FPU state (such as pending exceptions, legacy x87
> running
> code, NaN registers or weird FPU control word settings) we might have
> interrupted
> either.
>
> But I could be wrong, it should be checked whether this sequence is safe.
> Worst-case we might have to save/restore the FPU control and tag words - but
> those
> operations should still be much faster than a full XSAVE/XRSTOR pair.
Fair enough.
> 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.
Thanks,
tglx
