On Fri, Sep 15, 2023, Yan Zhao wrote:

On Wed, Sep 13, 2023 at 06:55:16PM -0700, Sean Christopherson wrote:
....

quoted

+static void kvm_mmu_prepare_memory_fault_exit(struct kvm_vcpu *vcpu,
+					      struct kvm_page_fault *fault)
+{
+	kvm_prepare_memory_fault_exit(vcpu, fault->gfn << PAGE_SHIFT,
+				      PAGE_SIZE, fault->write, fault->exec,
+				      fault->is_private);
+}
+
+static int kvm_faultin_pfn_private(struct kvm_vcpu *vcpu,
+				   struct kvm_page_fault *fault)
+{
+	int max_order, r;
+
+	if (!kvm_slot_can_be_private(fault->slot)) {
+		kvm_mmu_prepare_memory_fault_exit(vcpu, fault);
+		return -EFAULT;
+	}
+
+	r = kvm_gmem_get_pfn(vcpu->kvm, fault->slot, fault->gfn, &fault->pfn,
+			     &max_order);
+	if (r) {
+		kvm_mmu_prepare_memory_fault_exit(vcpu, fault);
+		return r;
+	}
+
+	fault->max_level = min(kvm_max_level_for_order(max_order),
+			       fault->max_level);
+	fault->map_writable = !(fault->slot->flags & KVM_MEM_READONLY);
+
+	return RET_PF_CONTINUE;
+}
+
 static int __kvm_faultin_pfn(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault)
 {
 	struct kvm_memory_slot *slot = fault->slot;

@@ -4293,6 +4356,14 @@ static int __kvm_faultin_pfn(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault
 			return RET_PF_EMULATE;
 	}
 
+	if (fault->is_private != kvm_mem_is_private(vcpu->kvm, fault->gfn)) {

In patch 21,
fault->is_private is set as:
	".is_private = kvm_mem_is_private(vcpu->kvm, cr2_or_gpa >> PAGE_SHIFT)",
then, the inequality here means memory attribute has been updated after
last check.
So, why an exit to user space for converting is required instead of a mere retry?

Or, is it because how .is_private is assigned in patch 21 is subjected to change
in future? 

This.  Retrying on SNP or TDX would hang the guest.  I suppose we could special
case VMs where .is_private is derived from the memory attributes, but the
SW_PROTECTED_VM type is primary a development vehicle at this point.  I'd like to
have it mimic SNP/TDX as much as possible; performance is a secondary concern.

E.g. userspace needs to be prepared for "spurious" exits due to races on SNP and
TDX, which this can theoretically exercise.  Though the window is quite small so
I doubt that'll actually happen in practice; which of course also makes it less
important to retry instead of exiting.

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