On Tue, Jul 16, 2024, Jason Gunthorpe wrote: > On Tue, Jul 16, 2024 at 09:03:00AM -0700, Sean Christopherson wrote: > > > > + To support huge pages, guest_memfd will take ownership of the hugepages, and > > > provide interested parties (userspace, KVM, iommu) with pages to be used. > > > + guest_memfd will track usage of (sub)pages, for both private and shared > > > memory > > > + Pages will be broken into smaller (probably 4K) chunks at creation time to > > > simplify implementation (as opposed to splitting at runtime when private to > > > shared conversion is requested by the guest) > > > > FWIW, I doubt we'll ever release a version with mmap()+guest_memfd support that > > shatters pages at creation. I can see it being an intermediate step, e.g. to > > prove correctness and provide a bisection point, but shattering hugepages at > > creation would effectively make hugepage support useless. > > Why? If the private memory retains its contiguity seperately but the > struct pages are removed from the vmemmap, what is the downside? Oooh, you're talking about shattering only the host userspace mappings. Now I understand why there was a bit of a disconnect, I was thinking you (hand-wavy everyone) were saying that KVM would immediately shatter its own mappings too. > As I understand it the point is to give a large contiguous range to > the private world and use only 4k pages to give the hypervisor world > access to limited amounts of the memory. > > Is there a reason that not having the shared memory elevated to higher > contiguity a deal breaker? Nope. I'm sure someone will ask for it sooner than later, but definitely not a must have.
