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SubjectRe: [PATCH 05/16] KVM: x86/mmu: Use synthetic page fault error code to indicate private faults
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On Thu, Feb 29, 2024, Kai Huang wrote:
>
> > diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> > index 408969ac1291..7807bdcd87e8 100644
> > --- a/arch/x86/kvm/mmu/mmu.c
> > +++ b/arch/x86/kvm/mmu/mmu.c
> > @@ -5839,19 +5839,31 @@ int noinline kvm_mmu_page_fault(struct kvm_vcpu *vcpu, gpa_t cr2_or_gpa, u64 err
> > bool direct = vcpu->arch.mmu->root_role.direct;
> >
> > /*
> > - * IMPLICIT_ACCESS is a KVM-defined flag used to correctly perform SMAP
> > - * checks when emulating instructions that triggers implicit access.
> > * WARN if hardware generates a fault with an error code that collides
> > - * with the KVM-defined value. Clear the flag and continue on, i.e.
> > - * don't terminate the VM, as KVM can't possibly be relying on a flag
> > - * that KVM doesn't know about.
> > + * with KVM-defined sythentic flags. Clear the flags and continue on,
> > + * i.e. don't terminate the VM, as KVM can't possibly be relying on a
> > + * flag that KVM doesn't know about.
> > */
> > - if (WARN_ON_ONCE(error_code & PFERR_IMPLICIT_ACCESS))
> > - error_code &= ~PFERR_IMPLICIT_ACCESS;
> > + if (WARN_ON_ONCE(error_code & PFERR_SYNTHETIC_MASK))
> > + error_code &= ~PFERR_SYNTHETIC_MASK;
> >
>
> Hmm.. I thought for TDX the caller -- handle_ept_violation() -- should
> explicitly set the PFERR_PRIVATE_ACCESS so that here the fault handler can
> figure out the fault is private.
>
> Now it seems the caller should never pass PFERR_PRIVATE_ACCESS, then ...
>
> > if (WARN_ON_ONCE(!VALID_PAGE(vcpu->arch.mmu->root.hpa)))
> > return RET_PF_RETRY;
> >
> > + /*
> > + * Except for reserved faults (emulated MMIO is shared-only), set the
> > + * private flag for software-protected VMs based on the gfn's current
> > + * attributes, which are the source of truth for such VMs. Note, this
> > + * wrong for nested MMUs as the GPA is an L2 GPA, but KVM doesn't
> > + * currently supported nested virtualization (among many other things)
> > + * for software-protected VMs.
> > + */
> > + if (IS_ENABLED(CONFIG_KVM_SW_PROTECTED_VM) &&
> > + !(error_code & PFERR_RSVD_MASK) &&
> > + vcpu->kvm->arch.vm_type == KVM_X86_SW_PROTECTED_VM &&
> > + kvm_mem_is_private(vcpu->kvm, gpa_to_gfn(cr2_or_gpa)))
> > + error_code |= PFERR_PRIVATE_ACCESS;
> > +
> >
>
> ... I am wondering how we figure out whether a fault is private for TDX?

Read the next few patches :-)

The sanity check gets moved to the legacy #PF handler (any error code with bits
63:32!=0 yells) and SVM's #NPF handler (error code with synthetic bits set yells),
leaving VMX free and clear to stuff PFERR_PRIVATE_ACCESS as appropriate.

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