Re: [PATCH v4 06/21] x86/fpu: Ignore APX when copying from/to guest FPU

[Paolo Bonzini]

On 5/14/26 19:33, Paolo Bonzini wrote:
> > If we _just_ look at making a good, consistent ABI, I don't think
> > XFEATURE_APX is special. The registers can be managed by XSAVE. There's
> > a KVM XSAVE ABI. Why would *this* feature be special from all of the others?
>
> Indeed, it's not special from the userspace point of view.
> KVM_GET_XSAVE and KVM_SET_XSAVE *do* include APX, but the copy needs
> to be done from struct kvm_vcpu rather than from the guest FPU state
> (which as of this series means arch/x86/kvm/ instead of
> arch/x86/fpu/core/).
>
> This is in the commit message, but it might be easy to miss:
> "Therefore, let KVM handle the APX feature on its own *when executing
> ioctls to get and set the virtual machine's XSAVE state*" (emphasis
> added).
>
> > I don't think the fact that the kernel is using those registers as GPRs
> > changes what makes a good ABI. That's a kernel-internal implementation
> > detail.
> >
> > Now, that's not to say we should ignore kernel complexity. Maybe making
> > a nice ABI is so nasty to the kernel internals that we shouldn't bother.
> > I _think_ that's what you're suggesting.
>
> No no, I'm only suggesting to keep the complexity out of FPU code
> until we have an idea of what the needs are for non-guest FPU state.

Hi Dave, can you confirm that this is okay for you?

Paolo