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

[Sean Christopherson <seanjc@google.com>]

On Tue, Apr 07, 2026, Paolo Bonzini wrote:
> Il mar 7 apr 2026, 00:00 Sean Christopherson <seanjc@google.com> 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.

/facepalm

I got so focused on register state that I completely forgot about actually
using the registers...