[RFC PATCH 0/5] running kernel mode SIMD with softirqs disabled

[Peter Zijlstra]

On Fri, Dec 18, 2020 at 06:01:01PM +0100, Ard Biesheuvel wrote:
> [ TL;DR for the non-ARM folks on CC: disabling softirq processing when using
>   SIMD in kernel mode could reduce complexity and improve performance, but we
>   need to decide whether we can do this, and how much softirq processing
>   latency we can tolerate. If we can find a satisfactory solution for this,
>   we might do the same for x86 and 32-bit ARM as well. ]

> - could we do the same on x86, now that kernel_fpu_begin/end is no longer
>   expensive?

Can't we simply save/restore the relevant register set?

So something like (note amluto was wanting to add a regset argument):

	<task>
	kernel_fpu_begin(MMX)
		<SIRQ>
		kernel_fpu_begin(SSE)
		kernel_fpu_end();
		</SIRQ>
	...
	kernel_fpu_end()

Would have to save the MMX regs on first SIRQ invocation of
kernel_fpu_begin(), and then have softirq context termination </SIRQ>
above, restore it.

I mean, we already do much the same for the first kernel_fpu_begin(),
that has to save the user registers, which will be restore when we go
back to userspace.

So why not do exactly the same for softirq context?