On Fri, Oct 24, 2025, Ackerley Tng wrote:

Sean Christopherson [off-list ref] writes:

quoted

quoted

[...snip...]

I've been thinking more about this:

  #ifdef CONFIG_KVM_VM_MEMORY_ATTRIBUTES
  	case KVM_CAP_MEMORY_ATTRIBUTES2:
  	case KVM_CAP_MEMORY_ATTRIBUTES:
  		if (!vm_memory_attributes)
  			return 0;
  
  		return kvm_supported_mem_attributes(kvm);
  #endif

And the purpose of adding KVM_CAP_MEMORY_ATTRIBUTES2 is that
KVM_CAP_MEMORY_ATTRIBUTES2 tells userspace that
KVM_SET_MEMORY_ATTRIBUTES2 is available iff there are valid
attributes.

(So there's still a purpose)

Without valid attributes, userspace can't tell if it should use
KVM_SET_MEMORY_ATTRIBUTES or the 2 version.

To do what?  If there are no attributes, userspace can't do anything useful anyways.

I also added KVM_CAP_GUEST_MEMFD_MEMORY_ATTRIBUTES, which tells
userspace the valid attributes when calling KVM_SET_MEMORY_ATTRIBUTES2
on a guest_memfd:

Ya, and that KVM_SET_MEMORY_ATTRIBUTES2 is supported on guest_memfd.

  #ifdef CONFIG_KVM_GUEST_MEMFD
  	case KVM_CAP_GUEST_MEMFD:
  		return 1;
  	case KVM_CAP_GUEST_MEMFD_FLAGS:
  		return kvm_gmem_get_supported_flags(kvm);
  	case KVM_CAP_GUEST_MEMFD_MEMORY_ATTRIBUTES:
  		if (vm_memory_attributes)
  			return 0;
  
  		return kvm_supported_mem_attributes(kvm);
  #endif
  
So to set memory attributes, userspace should

Userspace *can*.  User could also decide it only wants to support guest_memfd
attributes, e.g. because the platform admins controls the entire stack and built
their entire operation around in-place conversion.

  if (kvm_check_cap(KVM_CAP_GUEST_MEMFD_MEMORY_ATTRIBUTES) > 0)
	use KVM_SET_MEMORY_ATTRIBUTES2 with guest_memfd
  else if (kvm_check_cap(KVM_CAP_MEMORY_ATTRIBUTES2) > 0)
        use KVM_SET_MEMORY_ATTRIBUTES2 with VM fd
  else if (kvm_check_cap(KVM_CAP_MEMORY_ATTRIBUTES) > 0)
	use KVM_SET_MEMORY_ATTRIBUTES with VM fd
  else
	can't set memory attributes

Something like that?

More or else, ya.

In selftests there's this, when KVM_SET_USER_MEMORY_REGION2 was
introduced:

  #define TEST_REQUIRE_SET_USER_MEMORY_REGION2()			\
	__TEST_REQUIRE(kvm_has_cap(KVM_CAP_USER_MEMORY2),	\
		       "KVM selftests now require KVM_SET_USER_MEMORY_REGION2 (introduced in v6.8)")

But looks like there's no direct equivalent for the introduction of
KVM_SET_MEMORY_ATTRIBUTES2?

KVM_CAP_USER_MEMORY2 is the equivalent.

There's was no need to enumerate anything beyond yes/no, because
SET_USER_MEMORY_REGION2 didn't introduce new flags, it expanded the size of the
structure passed in from userspace so that KVM_CAP_GUEST_MEMFD could be introduced
without breaking backwards compatibility.

The closest would be to add a TEST_REQUIRE_VALID_ATTRIBUTES() which
checks KVM_CAP_MEMORY_ATTRIBUTES2 or
KVM_CAP_GUEST_MEMFD_MEMORY_ATTRIBUTES before making the vm or
guest_memfd ioctl respsectively.

Yes.  This is what I did in my (never posted, but functional) version:

@@ -486,6 +488,7 @@ struct kvm_vm *__vm_create(struct vm_shape shape, uint32_t nr_runnable_vcpus,
        }
        guest_rng = new_guest_random_state(guest_random_seed);
        sync_global_to_guest(vm, guest_rng);
+       sync_global_to_guest(vm, kvm_has_gmem_attributes);
 
        kvm_arch_vm_post_create(vm, nr_runnable_vcpus);
 

@@ -2319,6 +2333,8 @@ void __attribute((constructor)) kvm_selftest_init(void)
        guest_random_seed = last_guest_seed = random();
        pr_info("Random seed: 0x%x\n", guest_random_seed);
 
+       kvm_has_gmem_attributes = kvm_has_cap(KVM_CAP_GUEST_MEMFD_MEMORY_ATTRIBUTES);
+
        kvm_selftest_arch_init();
 }
 

That way the core library code can pivot on gmem vs. VM attributes without having
to rely on tests to define anything.  E.g.

static inline void vm_mem_set_memory_attributes(struct kvm_vm *vm, uint64_t gpa,
						uint64_t size, uint64_t attrs)
{
	if (kvm_has_gmem_attributes) {
		off_t fd_offset;
		uint64_t len;
		int fd;

		fd = kvm_gpa_to_guest_memfd(vm, gpa, &fd_offset, &len);
		TEST_ASSERT(len >= size, "Setting attributes beyond the length of a guest_memfd");
		gmem_set_memory_attributes(fd, fd_offset, size, attrs);
	} else {
		vm_set_memory_attributes(vm, gpa, size, attrs);
	}
}