Re: [PATCH v2 2/3] KVM: TDX: Do not retry locally when the retry is caused by invalid memslot

Sean Christopherson
2025-09-09T14:18:24Z

On Tue, Sep 09, 2025, Binbin Wu wrote:
> On 8/22/2025 3:05 PM, Yan Zhao wrote:
> > diff --git a/arch/x86/kvm/vmx/tdx.c b/arch/x86/kvm/vmx/tdx.c
> > index 6784aaaced87..de2c4bb36069 100644
> > --- a/arch/x86/kvm/vmx/tdx.c
> > +++ b/arch/x86/kvm/vmx/tdx.c
> > @@ -1992,6 +1992,11 @@ static int tdx_handle_ept_violation(struct kvm_vcpu *vcpu)
> > * blocked by TDs, false positives are inevitable i.e., KVM may re-enter
> > * the guest even if the IRQ/NMI can't be delivered.
> > *
> > + * Breaking out of the local retries if a retry is caused by faulting
> > + * in an invalid memslot (indicating the slot is under removal), so that
> > + * the slot removal will not be blocked due to waiting for releasing
> > + * SRCU lock in the VMExit handler.
> > + *
> > * Note: even without breaking out of local retries, zero-step
> > * mitigation may still occur due to
> > * - invoking of TDH.VP.ENTER after KVM_EXIT_MEMORY_FAULT,
> > @@ -2002,6 +2007,8 @@ static int tdx_handle_ept_violation(struct kvm_vcpu *vcpu)
> > * handle retries locally in their EPT violation handlers.
> > */
> > while (1) {
> > + struct kvm_memory_slot *slot;
> > +
> > ret = __vmx_handle_ept_violation(vcpu, gpa, exit_qual);
> > if (ret != RET_PF_RETRY || !local_retry)
> > @@ -2015,6 +2022,10 @@ static int tdx_handle_ept_violation(struct kvm_vcpu *vcpu)
> > break;
> > }
> > + slot = kvm_vcpu_gfn_to_memslot(vcpu, gpa_to_gfn(gpa));
> > + if (slot && slot->flags & KVM_MEMSLOT_INVALID)
>
> The slot couldn't be NULL here, right?

Uh, hmm. It could be NULL. If the memslot deletion starts concurrently with the
S-EPT violation, then the memslot could be transitioned to INVALID (prepared for
deletion) prior to the vCPU acquiring SRCU after the VM-Exit. Memslot deletion
could then assign to kvm->memslots with a NULL memslot.

vCPU DELETE
S-EPT Violation
Set KVM_MEMSLOT_INVALID
synchronize_srcu_expedited()
Acquire SRCU
__vmx_handle_ept_violation()
RET_PF_RETRY due to INVALID
Set memslot NULL
kvm_vcpu_gfn_to_memslot()