Re: [PATCH 1/6] KVM: guest_memfd: Add DEFAULT_SHARED flag, reject user page faults if not set

Ackerley Tng
2025-09-29T09:43:52Z

Fuad Tabba <tabba@google.com> writes:

> Hi Sean,
>
> On Fri, 26 Sept 2025 at 17:31, Sean Christopherson <seanjc@google.com> wrote:
>>
>> Add a guest_memfd flag to allow userspace to state that the underlying
>> memory should be configured to be shared by default, and reject user page
>> faults if the guest_memfd instance's memory isn't shared by default.
>> Because KVM doesn't yet support in-place private<=>shared conversions, all
>> guest_memfd memory effectively follows the default state.
>>
>> Alternatively, KVM could deduce the default state based on MMAP, which for
>> all intents and purposes is what KVM currently does. However, implicitly
>> deriving the default state based on MMAP will result in a messy ABI when
>> support for in-place conversions is added.
>>
>> For x86 CoCo VMs, which don't yet support MMAP, memory is currently private
>> by default (otherwise the memory would be unusable). If MMAP implies
>> memory is shared by default, then the default state for CoCo VMs will vary
>> based on MMAP, and from userspace's perspective, will change when in-place
>> conversion support is added. I.e. to maintain guest<=>host ABI, userspace
>> would need to immediately convert all memory from shared=>private, which
>> is both ugly and inefficient. The inefficiency could be avoided by adding
>> a flag to state that memory is _private_ by default, irrespective of MMAP,
>> but that would lead to an equally messy and hard to document ABI.
>>
>> Bite the bullet and immediately add a flag to control the default state so
>> that the effective behavior is explicit and straightforward.
>>

I like having this flag, but didn't propose this because I thought folks
depending on the default being shared (Patrick/Nikita) might have their
usage broken.

>> Fixes: 3d3a04fad25a ("KVM: Allow and advertise support for host mmap() on guest_memfd files")
>> Cc: David Hildenbrand <david@redhat.com>
>> Cc: Fuad Tabba <tabba@google.com>
>> Signed-off-by: Sean Christopherson <seanjc@google.com>
>> ---
>> Documentation/virt/kvm/api.rst | 10 ++++++++--
>> include/uapi/linux/kvm.h | 3 ++-
>> tools/testing/selftests/kvm/guest_memfd_test.c | 5 +++--
>> virt/kvm/guest_memfd.c | 6 +++++-
>> 4 files changed, 18 insertions(+), 6 deletions(-)
>>
>> diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
>> index c17a87a0a5ac..4dfe156bbe3c 100644
>> --- a/Documentation/virt/kvm/api.rst
>> +++ b/Documentation/virt/kvm/api.rst
>> @@ -6415,8 +6415,14 @@ guest_memfd range is not allowed (any number of memory regions can be bound to
>> a single guest_memfd file, but the bound ranges must not overlap).
>>
>> When the capability KVM_CAP_GUEST_MEMFD_MMAP is supported, the 'flags' field
>> -supports GUEST_MEMFD_FLAG_MMAP. Setting this flag on guest_memfd creation
>> -enables mmap() and faulting of guest_memfd memory to host userspace.
>> +supports GUEST_MEMFD_FLAG_MMAP and GUEST_MEMFD_FLAG_DEFAULT_SHARED. Setting
>
> There's an extra space between `and` and `GUEST_MEMFD_FLAG_DEFAULT_SHARED`.
>

+1 on this. Also, would you consider putting the concept of "at creation
time" or "at initialization time" into the name of the flag?

"Default" could be interpreted as "whenever a folio is allocated for
this guest_memfd", the memory the folio represents is by default
shared.

What we want to represent is that when the guest_memfd is created,
memory at all indices are initialized as shared.

Looking a bit further, when conversion is supported, if this flag is not
specified, then all the indices are initialized as private, right?

>> +the MMAP flag on guest_memfd creation enables mmap() and faulting of guest_memfd
>> +memory to host userspace (so long as the memory is currently shared). Setting
>> +DEFAULT_SHARED makes all guest_memfd memory shared by default (versus private
>> +by default). Note! Because KVM doesn't yet support in-place private<=>shared
>> +conversions, DEFAULT_SHARED must be specified in order to fault memory into
>> +userspace page tables. This limitation will go away when in-place conversions
>> +are supported.
>
> I think that a more accurate (and future proof) description of the
> mmap flag could be something along the lines of:
>

+1 on these suggestions, I agree that making the concepts of SHARED vs
MMAP orthogonal from the start is more future proof.

> + Setting GUEST_MEMFD_FLAG_MMAP enables using mmap() on the file descriptor.
>
> + Setting GUEST_MEMFD_FLAG_DEFAULT_SHARED makes all memory in the file shared
> + by default

See above, I'd prefer clarifying this as "at initialization time" or
something similar.

> , as opposed to private. Shared memory can be faulted into host
> + userspace page tables. Private memory cannot.
>
>> When the KVM MMU performs a PFN lookup to service a guest fault and the backing
>> guest_memfd has the GUEST_MEMFD_FLAG_MMAP set, then the fault will always be
>> diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
>> index 6efa98a57ec1..38a2c083b6aa 100644
>> --- a/include/uapi/linux/kvm.h
>> +++ b/include/uapi/linux/kvm.h
>> @@ -1599,7 +1599,8 @@ struct kvm_memory_attributes {
>> #define KVM_MEMORY_ATTRIBUTE_PRIVATE (1ULL << 3)
>>
>> #define KVM_CREATE_GUEST_MEMFD _IOWR(KVMIO, 0xd4, struct kvm_create_guest_memfd)
>> -#define GUEST_MEMFD_FLAG_MMAP (1ULL << 0)
>> +#define GUEST_MEMFD_FLAG_MMAP (1ULL << 0)
>> +#define GUEST_MEMFD_FLAG_DEFAULT_SHARED (1ULL << 1)
>>
>> struct kvm_create_guest_memfd {
>> __u64 size;
>> diff --git a/tools/testing/selftests/kvm/guest_memfd_test.c b/tools/testing/selftests/kvm/guest_memfd_test.c
>> index b3ca6737f304..81b11a958c7a 100644
>> --- a/tools/testing/selftests/kvm/guest_memfd_test.c
>> +++ b/tools/testing/selftests/kvm/guest_memfd_test.c
>> @@ -274,7 +274,7 @@ static void test_guest_memfd(unsigned long vm_type)
>> vm = vm_create_barebones_type(vm_type);
>>
>> if (vm_check_cap(vm, KVM_CAP_GUEST_MEMFD_MMAP))
>> - flags |= GUEST_MEMFD_FLAG_MMAP;
>> + flags |= GUEST_MEMFD_FLAG_MMAP | GUEST_MEMFD_FLAG_DEFAULT_SHARED;
>>
>> test_create_guest_memfd_multiple(vm);
>> test_create_guest_memfd_invalid_sizes(vm, flags, page_size);
>> @@ -337,7 +337,8 @@ static void test_guest_memfd_guest(void)
>> "Default VM type should always support guest_memfd mmap()");
>>
>> size = vm->page_size;
>> - fd = vm_create_guest_memfd(vm, size, GUEST_MEMFD_FLAG_MMAP);
>> + fd = vm_create_guest_memfd(vm, size, GUEST_MEMFD_FLAG_MMAP |
>> + GUEST_MEMFD_FLAG_DEFAULT_SHARED);
>> vm_set_user_memory_region2(vm, slot, KVM_MEM_GUEST_MEMFD, gpa, size, NULL, fd, 0);
>>
>> mem = mmap(NULL, size, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
>> diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c
>> index 08a6bc7d25b6..19f05a45be04 100644
>> --- a/virt/kvm/guest_memfd.c
>> +++ b/virt/kvm/guest_memfd.c
>> @@ -328,6 +328,9 @@ static vm_fault_t kvm_gmem_fault_user_mapping(struct vm_fault *vmf)
>> if (((loff_t)vmf->pgoff << PAGE_SHIFT) >= i_size_read(inode))
>> return VM_FAULT_SIGBUS;
>>
>> + if (!((u64)inode->i_private & GUEST_MEMFD_FLAG_DEFAULT_SHARED))
>> + return VM_FAULT_SIGBUS;
>> +
>> folio = kvm_gmem_get_folio(inode, vmf->pgoff);
>> if (IS_ERR(folio)) {
>> int err = PTR_ERR(folio);
>> @@ -525,7 +528,8 @@ int kvm_gmem_create(struct kvm *kvm, struct kvm_create_guest_memfd *args)
>> u64 valid_flags = 0;
>>
>> if (kvm_arch_supports_gmem_mmap(kvm))
>> - valid_flags |= GUEST_MEMFD_FLAG_MMAP;
>> + valid_flags |= GUEST_MEMFD_FLAG_MMAP |
>> + GUEST_MEMFD_FLAG_DEFAULT_SHARED;
>
> At least for now, GUEST_MEMFD_FLAG_DEFAULT_SHARED and
> GUEST_MEMFD_FLAG_MMAP don't make sense without each other. Is it worth
> checking for that, at least until we have in-place conversion? Having
> only GUEST_MEMFD_FLAG_DEFAULT_SHARED set, but GUEST_MEMFD_FLAG_MMAP,
> isn't a useful combination.
>

I think it's okay to have the two flags be orthogonal from the start.

Reviewed-by: Ackerley Tng <ackerleytng@google.com>

> That said, these are all nits, I'll leave it to you. With that:
>
> Reviewed-by: Fuad Tabba <tabba@google.com>
> Tested-by: Fuad Tabba <tabba@google.com>
>
> Cheers,
> /fuad
>
>
>
>>
>> if (flags & ~valid_flags)
>> return -EINVAL;
>> --
>> 2.51.0.536.g15c5d4f767-goog
>>