On Wed, Aug 03, 2022 at 03:51:24PM +0000, Sean Christopherson wrote: > On Wed, Aug 03, 2022, Chao Peng wrote: > > 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. > > My mental model breaks things down slightly differently, though the end result is > more or less the same. > > After this series, there will be two types of memory: private and "regular" (I'm > trying to avoid "shared"). "Regular" memory is always hva-based (userspace_addr), > and private always fd-based (fd+offset). > > In the future, if we want to support fd-based memory for "regular" memory, then > as you said we'd need to add a new flag, and a new fd+offset pair. > > At that point, we'd have two new (relatively to current) flags: > > KVM_MEM_PRIVATE_FD_VALID > KVM_MEM_FD_VALID > > along with two new pairs of fd+offset (private_* and "regular"). Mapping those > to your above list: I previously thought we could reuse the private_fd (name should be changed) for regular VM as well so only need one pair of fd+offset, the meaning of the fd can be decided by the flag. But introducing two pairs of them may support extra usages like one fd for regular memory and another private_fd for private memory, though unsure this is a useful configuration. > > 1. Neither *_FD_VALID flag set. > 2a. Both PRIVATE_FD_VALID and FD_VALID are set > 2b. FD_VALID is set and the VM doesn't support private memory > 3. Only PRIVATE_FD_VALID is set (which private memory support in the VM). > > Thus, "regular" VMs can't have a mix in a single memslot because they can't use > private memory. > > > There is no direct relationship between unmappable and fd-based since > > even fd-based can also be mappable for regular VM? Hmm, yes, for private memory we have special treatment in page fault handler and that is not applied to regular VM. > > Yep. > > > > 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? > > Hmm, no. After working through this, I think it's safe to say KVM_MEM_USER_UNMAPPABLE > is bad name because we could end up with "regular" memory that's backed by an > inaccessible (unmappable) file. > > One alternative would be to call it KVM_MEM_PROTECTED. That shouldn't cause > problems for the known use of "private" (TDX and SNP), and it gives us a little > wiggle room, e.g. if we ever get a use case where VMs can share memory that is > otherwise protected. > > That's a pretty big "if" though, and odds are good we'd need more memslot flags and > fd+offset pairs to allow differentiating "private" vs. "protected-shared" without > forcing userspace to punch holes in memslots, so I don't know that hedging now will > buy us anything. > > So I'd say that if people think KVM_MEM_PRIVATE brings additional and meaningful > clarity over KVM_MEM_PROTECTECD, then lets go with PRIVATE. But if PROTECTED is > just as good, go with PROTECTED as it gives us a wee bit of wiggle room for the > future. Then I'd stay with PRIVATE. > > Note, regardless of what name we settle on, I think it makes to do the > KVM_PRIVATE_MEM_SLOTS => KVM_INTERNAL_MEM_SLOTS rename. Agreed. Chao
