Re: [PATCH v6 0/8] KVM: mm: fd-based approach for supporting KVM guest private memory

[Sean Christopherson <seanjc@google.com>]

On Wed, Jun 15, 2022, Chao Peng wrote:
> On Tue, Jun 14, 2022 at 01:59:41PM -0700, Andy Lutomirski wrote:
> > On Tue, Jun 14, 2022 at 12:09 PM Sean Christopherson <seanjc@google.com> wrote:
> > >
> > > On Tue, Jun 14, 2022, Andy Lutomirski wrote:
> > > > This patch series is fairly close to implementing a rather more
> > > > efficient solution.  I'm not familiar enough with hypervisor userspace
> > > > to really know if this would work, but:
> > > >
> > > > What if shared guest memory could also be file-backed, either in the
> > > > same fd or with a second fd covering the shared portion of a memslot?
> > > > This would allow changes to the backing store (punching holes, etc) to
> > > > be some without mmap_lock or host-userspace TLB flushes?  Depending on
> > > > what the guest is doing with its shared memory, userspace might need
> > > > the memory mapped or it might not.
> > >
> > > That's what I'm angling for with the F_SEAL_FAULT_ALLOCATIONS idea.  The issue,
> > > unless I'm misreading code, is that punching a hole in the shared memory backing
> > > store doesn't prevent reallocating that hole on fault, i.e. a helper process that
> > > keeps a valid mapping of guest shared memory can silently fill the hole.
> > >
> > > What we're hoping to achieve is a way to prevent allocating memory without a very
> > > explicit action from userspace, e.g. fallocate().
> > 
> > Ah, I misunderstood.  I thought your goal was to mmap it and prevent
> > page faults from allocating.

I don't think you misunderstood, that's also one of the goals.  The use case is
that multiple processes in the host mmap() guest memory, and we'd like to be able
to punch a hole without having to rendezvous with all processes and also to prevent
an unintentional re-allocation.

> I think we still need the mmap, but want to prevent allocating when
> userspace touches previously mmaped area that has never filled the page.

Yes, or if a chunk was filled at some point but then was removed via PUNCH_HOLE.

> I don't have clear answer if other operations like read/write should be
> also prevented (probably yes). And only after an explicit fallocate() to
> allocate the page these operations would act normally.

I always forget about read/write.  I believe reads should be ok, the semantics of
holes are that they return zeros, i.e. can use ZERO_PAGE() and not allocate a new
backing page.  Not sure what to do about writes though.  Allocating on direct writes
might be ok for our use case, but that could also result in a rather wierd API.

> > It is indeed the case (and has been since before quite a few of us
> > were born) that a hole in a sparse file is logically just a bunch of
> > zeros.  A way to make a file for which a hole is an actual hole seems
> > like it would solve this problem nicely.  It could also be solved more
> > specifically for KVM by making sure that the private/shared mode that
> > userspace programs is strict enough to prevent accidental allocations
> > -- if a GPA is definitively private, shared, neither, or (potentially,
> > on TDX only) both, then a page that *isn't* shared will never be
> > accidentally allocated by KVM.
> 
> KVM is clever enough to not allocate since it knows a GPA is shared or
> not. This case it's the host userspace that can cause the allocating and
> is too complex to check on every access from guest.

Yes, KVM is not in the picture at all.  KVM won't trigger allocation, but KVM also
is not in a position to prevent userspace from touching memory.

> > If the shared backing is not mmapped,
> > it also won't be accidentally allocated by host userspace on a stray
> > or careless write.
> 
> As said above, mmap is still prefered, otherwise too many changes are
> needed for usespace VMM.

Forcing userspace to change doesn't bother me too much, the biggest concern is
having to take mmap_lock for write in a per-host process.