On Wed, Jul 01, 2026, Xiaoyao Li wrote:

On 6/19/2026 8:31 AM, Ackerley Tng via B4 Relay wrote:

quoted

@@ -4969,6 +4973,11 @@ static int kvm_vm_ioctl_check_extension_generic(struct kvm *kvm, long arg)
  		return 1;
  	case KVM_CAP_GUEST_MEMFD_FLAGS:
  		return kvm_gmem_get_supported_flags(kvm);
+	case KVM_CAP_GUEST_MEMFD_MEMORY_ATTRIBUTES:
+		if (!gmem_in_place_conversion || !kvm_supports_private_mem(kvm))
+			return 0;
+
+		return KVM_MEMORY_ATTRIBUTE_PRIVATE;
  #endif
  	default:
  		break;

this looks inconsistent with the

	case KVM_SET_MEMORY_ATTRIBUTES2:
		if (!gmem_in_place_conversion)
			return -ENOTTY;

Well, the check of

	if (!kvm_arch_has_private_mem(f->kvm))
		return -EINVAL;

is buried in the following kvm_gmem_set_attributes(). How about moving of
kvm_arch_has_private_mem() check to put it along with
gmem_in_place_conversion check in kvm_gmem_ioctl() in Patch 13?

Me confused, patch 13 already adds the kvm_arch_has_private_mem() in
kvm_gmem_set_attributes().

That said, the ordering here is wonky and misleading.  A cursory read of the series
would make one think that waiting to advertise KVM_CAP_GUEST_MEMFD_MEMORY_ATTRIBUTES
makes it safe/ok for KVM to plumb in support for KVM_SET_MEMORY_ATTRIBUTES2 over
multiple patches.  But that's not actually true, because the ioctl becomes live
the instant the code exists, userspace doesn't need to wait for KVM to formally
advertise support.

To further confuse matters, it is actually safe/ok to iteratively add support,
because it's all effectively dead code until "Let userspace disable per-VM mem
attributes, enable per-gmem attributes".

So, I think we should go a step further than what I think Xiaoyao is suggesting,
and fully squash patch 17 into patch 13.  That way the reader doesn't have to jump
through as many mental hoops to piece together what is happening.  It'll obviously
be a bigger patch, but should be easier to review/understand overall.

Oh, and that combined patch should carve out error_offset straightaway, so that
the full uAPI can be reviewed in a single patch.