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

David Hildenbrand
2025-09-29T09:01:59Z

On 29.09.25 10:57, Fuad Tabba wrote:
> Hi David.
>
> On Mon, 29 Sept 2025 at 09:38, David Hildenbrand <david@redhat.com> wrote:
>>
>> On 26.09.25 18:31, Sean Christopherson 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.
>>
>> I recall we discussed exactly that in the past (e.g., on April 17) in the call:
>>
>> "Current plan:
>> * guest_memfd creation flag to specify “all memory starts as shared”
>> * Compatible with the old behavior where all memory started as private
>> * Initially, only these can be mmap (no in-place conversion)
>> "
>>
>>>
>>> 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.
>>
>> I don't recall the details, but I faintly remember that we discussed later that with
>> mmap support, the default will be shared for now, and that no other flag would be
>> required for the time being.
>>
>> We could always add a "DEFAULT_PRIVATE" flag when we realize that we would have
>> to change the default later.
>
> I remember discussing this. For many confidential computing usecases,
> e.g., pKVM and TDX, it would make more sense for the default case to
> be private, since it's the more common state, and the initial state.
> It also makes sense since sharing is usually triggered by the guest.
> Ensuring that the initial state is private reduces the changes of the
> VMM forgetting to convert the memory to being private later on,
> potentially exposing all guest memory from the get go.
>
> I think it makes sense to clarify things now. Especially since with
> memory attributes, the default attribute is
> KVM_MEMORY_ATTRIBUTE_SHARED, which adds even more confusion.

Makes sense to me then, thanks.

--
Cheers

David / dhildenb