Sean Christopherson seanjc@google.com writes:
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);
I ported this [1] except for syncing this value to the guest, because I think the guest shouldn't need to know this information, the host should decide what to do. I think, if the guests really need to know this, the test itself can do the syncing.
[1] https://lore.kernel.org/all/5656d432df1217c08da0cc2694fd79948bfd686f.1760731...
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); } }