KVM: x86: Check EMULTYPE_WRITE_PF_TO_SP before unprotecting gfn

Don't bother unprotecting the target gfn if EMULTYPE_WRITE_PF_TO_SP is
set, as KVM will simply report the emulation failure to userspace.  This
will allow converting reexecute_instruction() to use
kvm_mmu_unprotect_gfn_instead_retry() instead of kvm_mmu_unprotect_page().

Link: https://lore.kernel.org/r/20240831001538.336683-17-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

Author: sean-jc

arch/x86/kvm/x86.c

@@ -8869,6 +8869,19 @@ static bool reexecute_instruction(struct kvm_vcpu *vcpu, gpa_t cr2_or_gpa,
 	if (!(emulation_type & EMULTYPE_ALLOW_RETRY_PF))
 		return false;
 
+	/*
+	 * If the failed instruction faulted on an access to page tables that
+	 * are used to translate any part of the instruction, KVM can't resolve
+	 * the issue by unprotecting the gfn, as zapping the shadow page will
+	 * result in the instruction taking a !PRESENT page fault and thus put
+	 * the vCPU into an infinite loop of page faults.  E.g. KVM will create
+	 * a SPTE and write-protect the gfn to resolve the !PRESENT fault, and
+	 * then zap the SPTE to unprotect the gfn, and then do it all over
+	 * again.  Report the error to userspace.
+	 */
+	if (emulation_type & EMULTYPE_WRITE_PF_TO_SP)
+		return false;
+
 	if (!vcpu->arch.mmu->root_role.direct) {
 		/*
 		 * Write permission should be allowed since only
@@ -8894,16 +8907,13 @@ static bool reexecute_instruction(struct kvm_vcpu *vcpu, gpa_t cr2_or_gpa,
 		kvm_mmu_unprotect_page(vcpu->kvm, gpa_to_gfn(gpa));
 
 	/*
-	 * If the failed instruction faulted on an access to page tables that
-	 * are used to translate any part of the instruction, KVM can't resolve
-	 * the issue by unprotecting the gfn, as zapping the shadow page will
-	 * result in the instruction taking a !PRESENT page fault and thus put
-	 * the vCPU into an infinite loop of page faults.  E.g. KVM will create
-	 * a SPTE and write-protect the gfn to resolve the !PRESENT fault, and
-	 * then zap the SPTE to unprotect the gfn, and then do it all over
-	 * again.  Report the error to userspace.
+	 * Retry even if _this_ vCPU didn't unprotect the gfn, as it's possible
+	 * all SPTEs were already zapped by a different task.  The alternative
+	 * is to report the error to userspace and likely terminate the guest,
+	 * and the last_retry_{eip,addr} checks will prevent retrying the page
+	 * fault indefinitely, i.e. there's nothing to lose by retrying.
 	 */
-	return !(emulation_type & EMULTYPE_WRITE_PF_TO_SP);
+	return true;
 }
 
 static int complete_emulated_mmio(struct kvm_vcpu *vcpu);