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

[Sean Christopherson <seanjc@google.com>]

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()