Re: [PATCH v7 11/14] KVM: Register/unregister the guest private memory regions

Chao Peng
2022-08-04T08:03:33Z

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