Re: [RFC PATCH v12 18/33] KVM: x86/mmu: Handle page fault for private memory

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On 9/15/2023 10:26 PM, Sean Christopherson wrote:
On Fri, Sep 15, 2023, Yan Zhao wrote:
On Wed, Sep 13, 2023 at 06:55:16PM -0700, Sean Christopherson wrote:
+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);
  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.
So when .is_private is derived from the memory attributes, and if I didn't miss anything, there is no explicit conversion mechanism introduced yet so far, does
it mean for pure sw-protected VM (withouth SNP/TDX), the page fault will be
handled according to the memory attributes setup by host/user vmm, no implicit
conversion will be triggered, right?

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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