Re: [RFC PATCH v1 07/37] KVM: Introduce KVM_SET_MEMORY_ATTRIBUTES2

[Sean Christopherson <seanjc@google.com>]

On Fri, Oct 24, 2025, Ackerley Tng wrote:
> Sean Christopherson <seanjc@google.com> writes:
> >> 
> >> [...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);
	}
}