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@xxxxxxxxxx> 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.
