Excerpts from Fabiano Rosas's message of October 16, 2021 11:45 pm:

Nicholas Piggin [off-list ref] writes:

quoted

This reduces the number of mtmsrd required to enable facility bits when
saving/restoring registers, by having the KVM code set all bits up front
rather than using individual facility functions that set their particular
MSR bits.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>

Reviewed-by: Fabiano Rosas <redacted>

Aside: at msr_check_and_set what's with MSR_VSX always being implicitly
set whenever MSR_FP is set? I get that it depends on MSR_FP, but if FP
always implies VSX, then you could stop setting MSR_VSX in this patch.

Good question, this seems to come from quite old code and is carried
forward. I did not immediately see why, might have been to avoid
another mtmsrd operation if we later want to set VSX.

But the rule seems to be to set MSR_VSX if both FP and VEC are set, so
this seems a bit odd. __msr_check_and_clear similarly clears VSX if we 
clear FP, but not if we clear VEC.

I might be good to remove that logic or turn it into warnings and make 
sure the callers do the right thing. Not sure.

Thanks,
Nick