KVM: guest_memfd: Always fault from guest_memfd if in-place conversion is enabled

If a guest_memfd memslot is created but the guest_memfd does not have the
GUEST_MEMFD_FLAG_MMAP set, KVM still fulfils guest faults by looking up the
memslot's userspace_addr.

Set KVM_MEMSLOT_GMEM_ONLY if in-place conversion is enabled so that the
guest_memfd's memory will be used for both shared and private memory. With
in-place conversion, guest_memfd will be the only backing memory for the
memslot.

No validation is performed to require userspace_addr to be a mapping from
the associated guest_memfd because even after validation, userspace is free
to remap something else at the provided userspace_addr.

userspace_addr will still be used by functions like kvm_read_guest(), and
if userspace_addr does not match up with the corresponding memory in the
memslot's guest_memfd (whether userspace_addr points to the wrong offset or
some non-guest_memfd memory, etc), that is a user error.

Requiring both shared and private memory to come from the only associated
guest_memfd simplifies invalidation in stage 2 page tables. On a PUNCH_HOLE
operation on a guest_memfd, the invalidation is now guaranteed to be
invalidating only memory mapped from the given guest_memfd.

Suggested-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Ackerley Tng <ackerleytng@google.com>
Reviewed-by: Fuad Tabba <fuad.tabba@linux.dev>
Reviewed-by: Binbin Wu <binbin.wu@linux.intel.com>
Reviewed-by: David Hildenbrand (Arm) <david@kernel.org>
Tested-by: Yan Zhao <yan.y.zhao@intel.com>
Link: https://patch.msgid.link/20260910-gmem-inplace-conversion-v13-12-dd6fbf94f4e1@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

0 files changed, 0 insertions, 0 deletions