Re: [RFC PATCH 04/12] vfio/pci: Allow MMIO regions to be exported through dma-buf

[Jason Gunthorpe]

On Wed, May 06, 2026 at 12:35:42PM +1000, Alexey Kardashevskiy wrote:
> Hi!
> 
> Let's reignite this topic.
> 
> I've been using these patches + QEMU side hacks for 6+ months. And it's been fine until I got a device where MSIX BAR is in a middle of another BAR marked as TEE in the TDISP interface report. And no trusted MSIX yet.
> 
> Every time QEMU mmaps a BAR - I request a dmabuf fd from VFIO in QEMU. Since mapping of an entire MSIX BAR is allowed by default, VFIORegion::nr_mmaps==1 and it is an entire BAR.
> 
> Problem: KVM memslot mismatches the dmabuf fd size 

Huh? kvm does not care about dmabuf at all? Are you running other
patches to hook kvm and dmabuf?

Putting a slice in a dmabuf is a well understood need for MSI, so I
expect whatever kvm dmabuf interface that gets merged to accomodate
this?

> Solution2: modify logic in VFIO dmabuf to allow multiple KVM memory
> slots per dmabuf. Now it is kvm_memory_slot::dmabuf_attach with no
> offset into the dmabuf and one kvm_vfio_dmabuf per dma_buf.

Yes, when kvm learns to take in a dmabuf it needs to take in a slice,
not the whole buf. Or you need to create multiple dmabufs with the
necessary slices from the VFIO. The upstream vfio dmabuf creation
allows creating it with a slice.

Jason