Re: [RFC PATCH 0/3] kernel: add support for 256-bit IO access

From: Thomas Gleixner
Date: Tue Mar 20 2018 - 04:38:54 EST


On Tue, 20 Mar 2018, Ingo Molnar wrote:
> * Thomas Gleixner <tglx@xxxxxxxxxxxxx> 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