On Tue, Aug 02, 2022 at 04:38:55PM +0000, Sean Christopherson wrote:
On Tue, Aug 02, 2022, Sean Christopherson wrote:
I think we should avoid UNMAPPABLE even on the KVM side of things for the core memslots functionality and instead be very literal, e.g.
KVM_HAS_FD_BASED_MEMSLOTS KVM_MEM_FD_VALID
We'll still need KVM_HAS_USER_UNMAPPABLE_MEMORY, but it won't be tied directly to the memslot. Decoupling the two thingis will require a bit of extra work, but the code impact should be quite small, e.g. explicitly query and propagate MEMFILE_F_USER_INACCESSIBLE to kvm_memory_slot to track if a memslot can be private. And unless I'm missing something, it won't require an additional memslot flag. The biggest oddity (if we don't also add KVM_MEM_PRIVATE) is that KVM would effectively ignore the hva for fd-based memslots for VM types that don't support private memory, i.e. userspace can't opt out of using the fd-based backing, but that doesn't seem like a deal breaker.
I actually love this idea. I don't mind adding extra code for potential usage other than confidential VMs if we can have a workable solution for it.
Hrm, but basing private memory on top of a generic FD_VALID would effectively require shared memory to use hva-based memslots for confidential VMs. That'd yield a very weird API, e.g. non-confidential VMs could be backed entirely by fd-based memslots, but confidential VMs would be forced to use hva-based memslots.
It would work if we can treat userspace_addr as optional for KVM_MEM_FD_VALID, e.g. userspace can opt in to decide whether needing the mappable part or not for a regular VM and we can enforce KVM for confidential VMs. But the u64 type of userspace_addr doesn't allow us to express a 'null' value so sounds like we will end up needing another flag anyway.
In concept, we could have three cofigurations here: 1. hva-only: without any flag and use userspace_addr; 2. fd-only: another new flag is needed and use fd/offset; 3. hva/fd mixed: both userspace_addr and fd/offset is effective. KVM_MEM_PRIVATE is a subset of it for confidential VMs. Not sure regular VM also wants this.
There is no direct relationship between unmappable and fd-based since even fd-based can also be mappable for regular VM?
Ignore this idea for now. If there's an actual use case for generic fd-based memory then we'll want a separate flag, fd, and offset, i.e. that support could be added independent of KVM_MEM_PRIVATE.
If we ignore this idea now (which I'm also fine), do you still think we need change KVM_MEM_PRIVATE to KVM_MEM_USER_UNMAPPBLE?
Thanks, Chao