Re: [PATCH v2 02/13] KVM: guest_memfd: Add INIT_SHARED flag, reject user page faults if not set

David Hildenbrand
2025-10-10T14:08:22Z

On 04.10.25 01:25, Sean Christopherson wrote:
> Add a guest_memfd flag to allow userspace to state that the underlying
> memory should be configured to be initialized as shared, and reject user
> page faults if the guest_memfd instance's memory isn't shared. Because
> KVM doesn't yet support in-place private<=>shared conversions, all
> guest_memfd memory effectively follows the initial state.
>
> Alternatively, KVM could deduce the initial 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.
>
> Fixes: 3d3a04fad25a ("KVM: Allow and advertise support for host mmap() on guest_memfd files")
> Cc: David Hildenbrand <david@redhat.com>
> Reviewed-by: Fuad Tabba <tabba@google.com>
> Tested-by: Fuad Tabba <tabba@google.com>
> Reviewed-by: Ackerley Tng <ackerleytng@google.com>
> Signed-off-by: Sean Christopherson <seanjc@google.com>
> ---

Reviewed-by: David Hildenbrand <david@redhat.com>

--
Cheers

David / dhildenb