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

[Ingo Molnar <mingo@kernel.org>]

* Thomas Gleixner <tglx@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.

Thanks,

	Ingo