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?

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