Fuad Tabba <tabba@xxxxxxxxxx> 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.
>
I think this explanation is a good summary, but this change seems to
make KVM take pages via guest_memfd functions for more than just
guest-private pages.
This change picks the guest_memfd fault path as long as the memslot has
an associated guest_memfd (kvm_slot_can_be_private()) and gmem was
configured to kvm_arch_gmem_supports_shared_mem().
For shared accesses, shouldn't KVM use the memslot's userspace_addr?
It's still possibly the same mmap-ed guest_memfd inode, but via
userspace_addr. And for special accesses from within KVM (e.g. clock),
maybe some other changes are required inside KVM, or those could also
use userspace_addr.
You mentioned that pKVM doesn't use
KVM_GENERIC_MEMORY_ATTRIBUTES/mem_attr_array, so perhaps the change
required here is that kvm_mem_is_private() should be updated to
kvm_slot_can_be_private() && whatever pKVM uses to determine if a gfn is
private?
So perhaps kvm_arch_gmem_supports_shared_mem() should be something like
kvm_arch_gmem_use_guest_memfd(), where x86 would override that to use
mem_attr_array if CONFIG_KVM_GENERIC_MEMORY_ATTRIBUTES is selected?
>
> Signed-off-by: Fuad Tabba <tabba@xxxxxxxxxx>
> ---
> 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 2d025b8ee20e..296f1d284d55 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 */
