Re: [PATCH 0/7] KVM: x86: APX reg prep work

[Paolo Bonzini]

Il mar 7 apr 2026, 00:00 Sean Christopherson <seanjc@xxxxxxxxxx> ha scritto:
>
> > > .  So unless I'm missing something (or hardware is flawed and lets the
> > > guest speculative consume R16-R31, which would be sad), it's perfectly safe to
> > > run the guest with host state in R16-R31.
> > >
> > > That would avoid pointlessly context switching 16 registers when APX is not being
> > > used by the guest, and would avoid having to write XCR0 in the fastpath.
> >
> > For now yes, but once/if the kernel starts using the registers there's
> > no way out of writing XCR0 for APX-disabled guests in the fast path.
>
> Why's that?  So long as KVM uses vcpu->arch.regs[R16-R31] as the source of truth
> when emulating anything, there's no danger of taking a #UD in the host due to
> accessing R16-R31 with XCR0.APX=0.

Yes I agree with that. But the unavoidable part is the XSETBV because
only the assembly code can run with XCR0.APX=0. As soon as you go back
to C, including during the fast path, you have to ensure XCR0.APX=1
again if the kernel is compiled with -mapxf.

For now, I agree that early_xcr0 isn't needed and you can run all the
time with XCR0.APX=0.

Paolo