Re: [PATCH v13 13/35] KVM: Introduce per-page memory attributes

[Chao Gao]

On Fri, Oct 27, 2023 at 11:21:55AM -0700, Sean Christopherson wrote:
>From: Chao Peng <chao.p.peng@xxxxxxxxxxxxxxx>
>
>In confidential computing usages, whether a page is private or shared is
>necessary information for KVM to perform operations like page fault
>handling, page zapping etc. There are other potential use cases for
>per-page memory attributes, e.g. to make memory read-only (or no-exec,
>or exec-only, etc.) without having to modify memslots.
>
>Introduce two ioctls (advertised by KVM_CAP_MEMORY_ATTRIBUTES) to allow
>userspace to operate on the per-page memory attributes.
>  - KVM_SET_MEMORY_ATTRIBUTES to set the per-page memory attributes to
>    a guest memory range.

>  - KVM_GET_SUPPORTED_MEMORY_ATTRIBUTES to return the KVM supported
>    memory attributes.

This ioctl() is already removed. So, the changelog is out-of-date and needs
an update.

>
>+
>+:Capability: KVM_CAP_MEMORY_ATTRIBUTES
>+:Architectures: x86
>+:Type: vm ioctl
>+:Parameters: struct kvm_memory_attributes(in)

					   ^ add one space here?


>+static bool kvm_pre_set_memory_attributes(struct kvm *kvm,
>+					  struct kvm_gfn_range *range)
>+{
>+	/*
>+	 * Unconditionally add the range to the invalidation set, regardless of
>+	 * whether or not the arch callback actually needs to zap SPTEs.  E.g.
>+	 * if KVM supports RWX attributes in the future and the attributes are
>+	 * going from R=>RW, zapping isn't strictly necessary.  Unconditionally
>+	 * adding the range allows KVM to require that MMU invalidations add at
>+	 * least one range between begin() and end(), e.g. allows KVM to detect
>+	 * bugs where the add() is missed.  Rexlaing the rule *might* be safe,

					    ^^^^^^^^ Relaxing

>@@ -4640,6 +4850,17 @@ static int kvm_vm_ioctl_check_extension_generic(struct kvm *kvm, long arg)
> 	case KVM_CAP_BINARY_STATS_FD:
> 	case KVM_CAP_SYSTEM_EVENT_DATA:
> 		return 1;
>+#ifdef CONFIG_KVM_GENERIC_MEMORY_ATTRIBUTES
>+	case KVM_CAP_MEMORY_ATTRIBUTES:
>+		u64 attrs = kvm_supported_mem_attributes(kvm);
>+
>+		r = -EFAULT;
>+		if (copy_to_user(argp, &attrs, sizeof(attrs)))
>+			goto out;
>+		r = 0;
>+		break;

This cannot work, e.g., no @argp in this function and is fixed by a later commit:

	fcbef1e5e5d2 ("KVM: Add KVM_CREATE_GUEST_MEMFD ioctl() for guest-specific backing memory")