Re: [PATCH v12 12/45] KVM: guest_memfd: Always fault from guest_memfd if in-place conversion is enabled

[Yan Zhao]

On Sun, Aug 30, 2026 at 05:25:13PM -0700, Ackerley Tng via B4 Relay wrote:
> From: Ackerley Tng <ackerleytng@xxxxxxxxxx>
> 
> 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@xxxxxxxxxx>
> Signed-off-by: Ackerley Tng <ackerleytng@xxxxxxxxxx>
> ---
>  Documentation/virt/kvm/api.rst | 22 ++++++++++++++--------
>  virt/kvm/guest_memfd.c         |  2 +-
>  2 files changed, 15 insertions(+), 9 deletions(-)
> 
> diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
> index 90a29424c54c8..668886f50024d 100644
> --- a/Documentation/virt/kvm/api.rst
> +++ b/Documentation/virt/kvm/api.rst
> @@ -6381,10 +6381,16 @@ mapping for userspace_addr is not required to be valid/populated at the time of
>  KVM_SET_USER_MEMORY_REGION2, e.g. shared memory can be lazily mapped/allocated
>  on-demand.
>  
> -When mapping a gfn into the guest, KVM selects shared vs. private, i.e consumes
> -userspace_addr vs. guest_memfd, based on the state in guest_memfd, which is the
> -sole authority on private vs. shared memory.  See :ref:`KVM_CREATE_GUEST_MEMFD`
> -to find out more about the creation-time shared/private status.
> +When mapping a gfn into the guest, guest faults are always serviced from
> +guest_memfd regardless of whether memory is shared or private.  KVM determines
> +shared vs. private based on the state in guest_memfd, which is the sole
> +authority on private vs. shared memory.  See :ref:`KVM_CREATE_GUEST_MEMFD` to
> +find out more about the creation-time shared/private status.
> +
> +userspace_addr is expected to be the mmap()-ed address corresponding to the
> +right offset within the guest_memfd. Any mismatch between userspace_addr and
> +guest_memfd is not validated and is a user error. userspace_addr is only used
> +for host-side guest accesses such as kvm_read_guest().
>  
>  If in-place conversion is disabled, KVM selects shared vs. private, i.e consumes
>  userspace_addr vs. guest_memfd, based on the gfn's KVM_MEMORY_ATTRIBUTE_PRIVATE
> @@ -6490,10 +6496,10 @@ specified via KVM_CREATE_GUEST_MEMFD.  Currently defined flags:
>                                 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
> -consumed from guest_memfd, regardless of whether it is a shared or a private
> -fault.
> +When the KVM MMU performs a PFN lookup to service a guest fault, the fault will
> +always be consumed from guest_memfd, regardless of whether it is a shared or a
> +private fault (unless in-place conversion is disabled and the backing
> +guest_memfd does not have the GUEST_MEMFD_FLAG_MMAP flag set).
>  
>  See KVM_SET_USER_MEMORY_REGION2 for additional details.
>  
> diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c
> index 0afe1468d2d9d..e41802944756b 100644
> --- a/virt/kvm/guest_memfd.c
> +++ b/virt/kvm/guest_memfd.c
> @@ -746,7 +746,7 @@ int kvm_gmem_bind(struct kvm *kvm, struct kvm_memory_slot *slot,
>  	 */
>  	WRITE_ONCE(slot->gmem.file, file);
>  	slot->gmem.pgoff = start;
> -	if (kvm_gmem_supports_mmap(inode))
> +	if (gmem_in_place_conversion || kvm_gmem_supports_mmap(inode))
>  		slot->flags |= KVM_MEMSLOT_GMEM_ONLY;
I like this change, which actually enforces in-place conversion -- when
gmem_in_place_conversion is true, if userspace sets slot->userspace_addr to a
different backend, the host and guest will no longer be able to access the same
backend.

However, since gmem_in_place_conversion is globally and statically specified, it
essentially disables the coexistence of VMs using out-of-place conversions when
gmem_in_place_conversion is true. Previously, VMs using out-of-place conversions
could still boot successfully as long as they switched to using per-gmem memory
attributes. Is this change intended?

On a separate note, is kvm_gmem_supports_mmap() still a necessary requirement
for setting KVM_MEMSLOT_GMEM_ONLY? When gmem_in_place_conversion is false, if
userspace mmap()s the gmem but sets slot->userspace_addr to a different backend,
should the KVM_MEMSLOT_GMEM_ONLY flag still be set?

>  
>  	xa_store_range(&f->bindings, start, end - 1, slot, GFP_KERNEL);
> 
> -- 
> 2.55.0.897.gb25b4bd76c-goog
> 
>