Re: [RFC PATCH 10/30] vfio/pci: Export vfio dma-buf specific info for importers

Jason Gunthorpe <[email protected]> writes:

> On Sat, Jul 11, 2026 at 06:01:31PM -0700, Ackerley Tng wrote:
>
>> In the course of a CoCo guest's operation, will the guest need to
>> convert between private/shared MMIO? Will the guest need some pages
>> shared and others private? If these are required operations, guest_memfd
>> already provides the tracking and is going to have a conversion ioctl
>> very soon. Instead of further extending dmabuf to track more things, how
>> about letting guest_memfd track it?
>
> Use another FD type was sort of my fallback if we couldn't get DMABUF
> into something workable. I'm kind of surprised to see guestmemfd
> proposed as the other FD, but I don't know much about its insides.
>

For now guest_memfd only supports 4K pages, so the interfaces we're
interested in are:

+ The .fault() handler, which core mm calls to service any page
  faults. For this, guest_memfd first checks if the page is shared, and
  if so, proceeds to return a struct page.
    + For VFIO, guest_memfd could call vfio_pci_mmap_huge_fault() and do
      vmf_insert_pfn().
+ The kvm_gmem_get_pfn() function, which kvm calls to get a pfn and a
  max_order to set up stage 2 page tables. Now guest_memfd returns
  folio_pfn(folio) and max_order = 0 since guest_memfd only supports
  PAGE_SIZE pages now.
    + For VFIO, guest_memfd would need some way to get the PFN, so
      probably something like dma_buf_get_pfn_unlocked() [1], but
      probably from VFIO instead of dmabuf. Perhaps pci_resource_start()
      is good enough to get a pfn.

[1] https://lore.kernel.org/all/[email protected]/

> If VFIO can create one

At guest_memfd creation time, guest_memfd is always created for a
KVM. This is the first place where it is bound to a kvm instance. This
is one place where it helps Yilun with Confidential VMs, so KVM can be
sure that the memory was meant for a specific CoCo kvm and not any VM.

I think it might be weird to have VFIO take a kvm fd to create a
guest_memfd?

I was thinking that the userspace VMM could do something like:

  int vfio_fd = open("/dev/vfio/devices/vfio0");
  int gmem_fd = ioctl(KVM_CREATE_GUEST_MEMFD, { .fd = vfio_fd });

and creating from a template fd would also disable/close the vfio_fd so
that the following will fail:

  addr = mmap(vfio_fd);  // Maybe this should fail too?
  *addr = 1;             // But this should definitely fail so that
                         // guest_memfd gets to block private writes.

> and fill it with MMIO physical addresses then
> maybe it is OK?
>
> Jason

What interface would be used fill a guest_memfd with MMIO physical
addresses?

When a vdev is associated with a vfio file, can some other vfio file be
created that can also do MMIO to the vdev's PFNs? Would I be able to set
something to block any future associations to those PFNs? This will help
guest_memfd to prevent any other writes to private PFNs.