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

[Binbin Wu]

On 8/31/2026 8:25 AM, 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>

Reviewed-by: Binbin Wu <binbin.wu@xxxxxxxxxxxxxxx>