Re: [PATCH v4 2/5] KVM: x86: Tell ->inject_page_fault() whether or a fault came from hardware

[Sean Christopherson <seanjc@google.com>]

On Tue, May 26, 2026, Yosry Ahmed wrote:
> On Fri, May 22, 2026 at 4:27 PM Sean Christopherson <seanjc@google.com> wrote:
> >
> > When injecting a page fault (including nested TDP faults into L1), tell the
> > injection routine whether or not the fault originated in hardware, i.e. if
> > KVM is effectively forwarding a fault it intercept.  For nested TDP fault
> > injection, KVM needs to grab PAGE_WALK vs. GUEST_FINAL information from the
> > VMCB/VMCS, _if_ the fault originated in hardware.
> >
> > No functional change intended (nothing uses the new param, yet...).
> >
> > Signed-off-by: Sean Christopherson <seanjc@google.com>
> > ---
> >  arch/x86/include/asm/kvm_host.h | 18 ++++++++++++++----
> >  arch/x86/kvm/mmu/paging_tmpl.h  |  2 +-
> >  arch/x86/kvm/svm/nested.c       |  3 ++-
> >  arch/x86/kvm/vmx/nested.c       |  3 ++-
> >  arch/x86/kvm/x86.c              | 16 +++++++++-------
> >  5 files changed, 28 insertions(+), 14 deletions(-)
> >
> > diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> > index 271bdd109a98..d11063c36f03 100644
> > --- a/arch/x86/include/asm/kvm_host.h
> > +++ b/arch/x86/include/asm/kvm_host.h
> > @@ -484,7 +484,8 @@ struct kvm_mmu {
> >         u64 (*get_pdptr)(struct kvm_vcpu *vcpu, int index);
> >         int (*page_fault)(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault);
> >         void (*inject_page_fault)(struct kvm_vcpu *vcpu,
> > -                                 struct x86_exception *fault);
> > +                                 struct x86_exception *fault,
> > +                                 bool from_hardware);
> 
> Probably a bit late to ask this question, but why do we need
> from_hardware (or the previous hardware_nested_page_fault) as opposed
> to just checking exit_code / exit_reason? Is it possible to get an
> NPF/EPT violation but then synthesize a different one into L1 rather
> than forwarding the one we got from HW?

Yes.  E.g. if access to emulated MMIO from L2 hit a !PRESENT fault (EPT Violation
or #NPF), e.g. because MMIO caching is disabled or it's the first time the GPA has
been accessed by L2, then KVM will enter the emulator.  If emulating the MMIO
access then hits a TDP fault, e.g. because L2 was accessing MMIO with a MOVQ
(memory-to-memory move), or because L1 has since unmapped the code stream, then
the TDP fault synthesized to L1 will not be the "same" fault the triggered the
VM-Exit.