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

[Thomas Gleixner]

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