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?
