On Fri, Jan 02, 2026, Vitaly Kuznetsov wrote:
> Sean Christopherson <seanjc@google.com> writes:
>
> > Add a build-time assertiont that Hyper-V's "enlightened" exit code is that,
> > same as the AMD-defined "Reserved for Host" exit code, mostly to help
> > readers connect the dots and understand why synthesizing a software-defined
> > exit code is safe/ok.
> >
> > Signed-off-by: Sean Christopherson <seanjc@google.com>
> > ---
> > arch/x86/kvm/svm/hyperv.c | 6 ++++++
> > 1 file changed, 6 insertions(+)
> >
> > diff --git a/arch/x86/kvm/svm/hyperv.c b/arch/x86/kvm/svm/hyperv.c
> > index 3ec580d687f5..4f24dcb45116 100644
> > --- a/arch/x86/kvm/svm/hyperv.c
> > +++ b/arch/x86/kvm/svm/hyperv.c
> > @@ -10,6 +10,12 @@ void svm_hv_inject_synthetic_vmexit_post_tlb_flush(struct kvm_vcpu *vcpu)
> > {
> > struct vcpu_svm *svm = to_svm(vcpu);
> >
> > + /*
> > + * The exit code used by Hyper-V for software-defined exits is reserved
> > + * by AMD specifically for such use cases.
> > + */
> > + BUILD_BUG_ON(HV_SVM_EXITCODE_ENL != SVM_EXIT_SW);
> > +
> > svm->vmcb->control.exit_code = HV_SVM_EXITCODE_ENL;
> > svm->vmcb->control.exit_info_1 = HV_SVM_ENL_EXITCODE_TRAP_AFTER_FLUSH;
> > svm->vmcb->control.exit_info_2 = 0;
>
> Reviewed-by: Vitaly Kuznetsov <vkuznets@redhat.com>
>
> Alternatively (or additionally?) to BUG_ON, I guess we could've
>
> #define HV_SVM_EXITCODE_ENL SVM_EXIT_SW
>
> unless including SVM's headers into include/hyperv/hvgdk.h is too big of
> a mess.
Heh, I had the same thought[*], but Wei pointed out that the definitions in hvgdk.h
mirror internal Microsoft headers:
On Fri, Nov 14, 2025, Wei Liu wrote:
> On Fri, Nov 14, 2025 at 07:22:41AM -0800, Sean Christopherson wrote:
> > On Fri, Nov 14, 2025, Michael Kelley wrote:
> > > From: Sean Christopherson <seanjc@google.com> Sent: Thursday, November 13, 2025 2:56 PM
> > > > @@ -281,7 +281,7 @@ struct hv_vmcb_enlightenments {
> > > > #define HV_VMCB_NESTED_ENLIGHTENMENTS 31
> > > >
> > > > /* Synthetic VM-Exit */
> > > > -#define HV_SVM_EXITCODE_ENL 0xf0000000
> > > > +#define HV_SVM_EXITCODE_ENL 0xf0000000u
> > >
> > > Is there a reason for making this Hyper-V code just "u", while
> > > making the SVM_VMGEXIT_* values "ull"? I don't think
> > > "u" vs. "ull" shouldn't make any difference when assigning to a
> > > u64, but the inconsistency piqued my interest ....
> >
> > I hedged and went for a more "minimal" change because it isn't KVM code, and at
> > the time because I thought the value isn't defined by the APM. Though looking
> > again at the APM, it does reserve that value for software
> >
> > F000_000h Unused Reserved for Host.
> >
> > and I can't find anything in the TLFS. Ah, my PDF copy is just stale, it's indeed
> > defined as a synthetic exit.
> >
> > https://learn.microsoft.com/en-us/virtualization/hyper-v-on-windows/tlfs/nested-virtualization#synthetic-vm-exit
> >
> > Anyways, I'm in favor of making HV_SVM_EXITCODE_ENL an ull, though part of me
> > wonders if we should do:
> >
> > #define HV_SVM_EXITCODE_ENL SVM_EXIT_SW
>
> I know this is very tempting, but these headers are supposed to mirror
> Microsoft's internal headers, so we would like to keep them
> self-contained for ease of tracking.
>
> It should be fine to add the "ull" suffix here. I briefly talked to a
> hypervisor developer and they agreed.
[*] https://lore.kernel.org/all/aRdJQQ7_j6RcHwjJ@google.com