linux-kernel - Re: [PATCH v2 8/8] KVM: SVM: Assert that Hyper-V's HV_SVM_EXITCODE_ENL == SVM_EXIT

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Message-ID: <aVveISeqIBPmZ7xW@google.com>
Date: Mon, 5 Jan 2026 07:52:01 -0800
From: Sean Christopherson <seanjc@...gle.com>
To: Vitaly Kuznetsov <vkuznets@...hat.com>
Cc: Paolo Bonzini <pbonzini@...hat.com>, "K. Y. Srinivasan" <kys@...rosoft.com>, 
	Haiyang Zhang <haiyangz@...rosoft.com>, Wei Liu <wei.liu@...nel.org>, 
	Dexuan Cui <decui@...rosoft.com>, Long Li <longli@...rosoft.com>, kvm@...r.kernel.org, 
	linux-hyperv@...r.kernel.org, linux-kernel@...r.kernel.org, 
	Jim Mattson <jmattson@...gle.com>, Yosry Ahmed <yosry.ahmed@...ux.dev>
Subject: Re: [PATCH v2 8/8] KVM: SVM: Assert that Hyper-V's
 HV_SVM_EXITCODE_ENL == SVM_EXIT_SW

On Fri, Jan 02, 2026, Vitaly Kuznetsov wrote:
> Sean Christopherson <seanjc@...gle.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@...gle.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@...hat.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@...gle.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