KVM: guest_memfd: Add INIT_SHARED flag, reject user page faults if not set

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>
Tested-by: Ackerley Tng <ackerleytng@google.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Link: https://lore.kernel.org/r/20251003232606.4070510-3-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

0 files changed, 0 insertions, 0 deletions