Re: [patch 13/31] x86/fpu: Move KVMs FPU swapping to FPU core

Paolo Bonzini
2021-10-14T06:50:46Z

On 13/10/21 16:06, Thomas Gleixner wrote:
>> - the guest value stored in vcpu->arch.
>>
>> - the "QEMU" value attached to host_fpu. This one only becomes zero if
>> QEMU requires AMX (which shouldn't happen).
>
> I don't think that makes sense.
>
> First of all, if QEMU wants to expose AMX to guests, then it has to ask
> for permission to do so as any other user space process. We're not going
> to make that special just because.

Hmm, I would have preferred if there was no need to enable AMX for the
QEMU FPU. But you're saying that guest_fpu needs to swap out to
current->thread.fpu if the guest is preempted, which would require
XFD=0; and affect QEMU operation as well.

In principle I don't like it very much; it would be nicer to say "you
enable it for QEMU itself via arch_prctl(ARCH_SET_STATE_ENABLE), and for
the guests via ioctl(KVM_SET_CPUID2)". But I can see why you want to
keep things simple, so it's not a strong objection at all.

> Anything else will just create more problems than it solves. Especially
> #NM handling (think nested guest) and the XFD_ERR additive behaviour
> will be a nasty playground and easy to get wrong.
>
> Not having that at all makes life way simpler, right?

It is simpler indeed, and it makes sense to start simple. I am not sure
if it will hold, but I agree it's better for the first implementation.

Paolo