Re: [PATCH v3 05/11] KVM: guest_memfd: Handle in-place shared memory as guest_memfd backed memory

[Ackerley Tng <ackerleytng@google.com>]

Fuad Tabba <tabba@google.com> writes:

> For VMs that allow sharing guest_memfd backed memory in-place,
> handle that memory the same as "private" guest_memfd memory. This
> means that faulting that memory in the host or in the guest will
> go through the guest_memfd subsystem.
>
> Note that the word "private" in the name of the function
> kvm_mem_is_private() doesn't necessarily indicate that the memory
> isn't shared, but is due to the history and evolution of
> guest_memfd and the various names it has received. In effect,
> this function is used to multiplex between the path of a normal
> page fault and the path of a guest_memfd backed page fault.
>

Thanks for this summary! It has always been confusing and this really
helps.

Is there any chance we could rename the functions in KVM, or maybe add a
comment at the function definitions? The name of the userspace flag will
have to remain, of course.

> Signed-off-by: Fuad Tabba <tabba@google.com>
> ---
>  include/linux/kvm_host.h | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> index 438aa3df3175..39fd6e35c723 100644
> --- a/include/linux/kvm_host.h
> +++ b/include/linux/kvm_host.h
> @@ -2521,7 +2521,8 @@ static inline bool kvm_mem_is_private(struct kvm *kvm, gfn_t gfn)
>  #else
>  static inline bool kvm_mem_is_private(struct kvm *kvm, gfn_t gfn)
>  {
> -	return false;
> +	return kvm_arch_gmem_supports_shared_mem(kvm) &&
> +	       kvm_slot_can_be_private(gfn_to_memslot(kvm, gfn));
>  }
>  #endif /* CONFIG_KVM_GENERIC_MEMORY_ATTRIBUTES */