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

[Alexey Kardashevskiy]

On 6/5/26 23:16, Jason Gunthorpe wrote:
> 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?

yup, 06/12 of this patchset.

> 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?

good to know.

> > 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.

true but either way dmabuf slicing will be directed by QEMU's msix-table emulation MR and this slicing needs to match the TDISP report so I'll have to teach QEMU these reports, right? I am worried if I miss something obvious, again. Thanks,


ps. I like nntp.lore.kernel.org very much for ability to dig out old stuff and then just reply to it :)

> Jason

--
Alexey