Messages in this thread | | | Date | Thu, 14 Oct 2021 08:50:38 +0200 | Subject | Re: [patch 13/31] x86/fpu: Move KVMs FPU swapping to FPU core | From | Paolo Bonzini <> |
| |
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
| |