Re: [PATCH v5 2/8] KVM: nSVM: Synthesize DecodeAssists EXITINFO for emulated intercepts

[Jim Mattson]

On Sat, Sep 5, 2026 at 7:24 PM Tina Zhang <zhang_wei@xxxxxxxxxxxxxx> wrote:
>
>
>
> On 9/5/2026 7:07 AM, Jim Mattson wrote:
> > On Mon, Aug 24, 2026 at 5:40 AM Tina Zhang <zhang_wei@xxxxxxxxxxxxxx> wrote:
> >>
> >> +
> >> +       vmcb->control.exit_info_1 = exit_info_1;
> >> +       vmcb->control.exit_info_2 = 0;
> >> +}
> >
> > Can this entire function be integrated into the existing switch
> > statement in svm_check_intercept()?
>
> Yes, that makes sense.
>
> >
> > And, if !guest_cpu_cap_has(vcpu, X86_FEATURE_DECODEASSISTS), should
> > exit_info_1 be cleared for these intercepts?
>
> My reading is that exit_info_1 is undefined for these intercepts when
> DecodeAssists is not exposed, and KVM previously left it untouched. I
> would therefore prefer to preserve the existing behavior and update
> exit_info_1 only when DecodeAssists is exposed. That said, do you see
> any reason why KVM should explicitly clear exit_info_1 when
> guest_cpu_cap_has(vcpu, X86_FEATURE_DECODEASSISTS) returns false?

You're right. The APM says, "Note that the contents of the EXITINFO1
and EXITINFO2 fields are undefined for intercepts where their use is
not indicated," so pre-Bulldozer CPUs would probably have left them
alone.