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

[Jason Gunthorpe <jgg@nvidia.com>]

On Tue, May 12, 2026 at 09:42:01AM +1000, Alexey Kardashevskiy wrote:

> > 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?
>
> Or TDISP devices are going to align MSIX BARs to 4K, and QEMU will
> do the same and it should "just work", and if it does not - the host
> won't crash. Can this work? Thanks,

Host crashing stuff is a different issue, I think the plan was to
revoke the entire MMIO space from userspace and remove it from the
kernel mapping. Entire because we don't want to parse the TDISP report
to figure out something more narrow.

Therefore there is no way the host can crash.

When qemu constructs the VM memory map it already has a scheme to
insert a hole for a SW emulated page for MSI. That will keep working
exactly as it is.

When the VM validates the MMIO the hole has to fall within a T=0 space
of the TDISP report or the VM will reject it.

This means devices need to have a T=0 hole around their MSI-X/etc
suitable for a 64K page size OS.

This is already the case, if a device mixes MSIx with other things
qemu will work but it becomes horribly slow and a little broken.

Jason