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

* Thomas Gleixner [off-list ref] wrote:

quoted

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