KVM: arm64: nv: Handle VNCR_EL2-triggered faults backed by guest_memfd

Handle faults for memslots backed by guest_memfd in arm64 nested
virtualization triggered by VNCR_EL2.

* Introduce is_gmem output parameter to kvm_translate_vncr(), indicating
  whether the faulted memory slot is backed by guest_memfd.

* Dispatch faults backed by guest_memfd to kvm_gmem_get_pfn().

* Update kvm_handle_vncr_abort() to handle potential guest_memfd errors.
  Some of the guest_memfd errors need to be handled by userspace instead
  of attempting to (implicitly) retry by returning to the guest.

Suggested-by: Marc Zyngier <[email protected]>
Reviewed-by: Marc Zyngier <[email protected]>
Signed-off-by: Fuad Tabba <[email protected]>
Signed-off-by: Sean Christopherson <[email protected]>
Message-ID: <[email protected]>
Signed-off-by: Paolo Bonzini <[email protected]>

Author: Fuad Tabba

arch/arm64/kvm/nested.c

@@ -1172,8 +1172,9 @@ static u64 read_vncr_el2(struct kvm_vcpu *vcpu)
 	return (u64)sign_extend64(__vcpu_sys_reg(vcpu, VNCR_EL2), 48);
 }
 
-static int kvm_translate_vncr(struct kvm_vcpu *vcpu)
+static int kvm_translate_vncr(struct kvm_vcpu *vcpu, bool *is_gmem)
 {
+	struct kvm_memory_slot *memslot;
 	bool write_fault, writable;
 	unsigned long mmu_seq;
 	struct vncr_tlb *vt;
@@ -1216,10 +1217,25 @@ static int kvm_translate_vncr(struct kvm_vcpu *vcpu)
 	smp_rmb();
 
 	gfn = vt->wr.pa >> PAGE_SHIFT;
-	pfn = kvm_faultin_pfn(vcpu, gfn, write_fault, &writable, &page);
-	if (is_error_noslot_pfn(pfn) || (write_fault && !writable))
+	memslot = gfn_to_memslot(vcpu->kvm, gfn);
+	if (!memslot)
 		return -EFAULT;
 
+	*is_gmem = kvm_slot_has_gmem(memslot);
+	if (!*is_gmem) {
+		pfn = __kvm_faultin_pfn(memslot, gfn, write_fault ? FOLL_WRITE : 0,
+					&writable, &page);
+		if (is_error_noslot_pfn(pfn) || (write_fault && !writable))
+			return -EFAULT;
+	} else {
+		ret = kvm_gmem_get_pfn(vcpu->kvm, memslot, gfn, &pfn, &page, NULL);
+		if (ret) {
+			kvm_prepare_memory_fault_exit(vcpu, vt->wr.pa, PAGE_SIZE,
+					      write_fault, false, false);
+			return ret;
+		}
+	}
+
 	scoped_guard(write_lock, &vcpu->kvm->mmu_lock) {
 		if (mmu_invalidate_retry(vcpu->kvm, mmu_seq))
 			return -EAGAIN;
@@ -1292,23 +1308,36 @@ int kvm_handle_vncr_abort(struct kvm_vcpu *vcpu)
 	if (esr_fsc_is_permission_fault(esr)) {
 		inject_vncr_perm(vcpu);
 	} else if (esr_fsc_is_translation_fault(esr)) {
-		bool valid;
+		bool valid, is_gmem = false;
 		int ret;
 
 		scoped_guard(read_lock, &vcpu->kvm->mmu_lock)
 			valid = kvm_vncr_tlb_lookup(vcpu);
 
 		if (!valid)
-			ret = kvm_translate_vncr(vcpu);
+			ret = kvm_translate_vncr(vcpu, &is_gmem);
 		else
 			ret = -EPERM;
 
 		switch (ret) {
 		case -EAGAIN:
-		case -ENOMEM:
 			/* Let's try again... */
 			break;
+		case -ENOMEM:
+			/*
+			 * For guest_memfd, this indicates that it failed to
+			 * create a folio to back the memory. Inform userspace.
+			 */
+			if (is_gmem)
+				return 0;
+			/* Otherwise, let's try again... */
+			break;
 		case -EFAULT:
+		case -EIO:
+		case -EHWPOISON:
+			if (is_gmem)
+				return 0;
+			fallthrough;
 		case -EINVAL:
 		case -ENOENT:
 		case -EACCES: