Re: [PATCH v5 09/14] PCI: portdrv: Suppress kernel DMA ownership auto-claiming

[Bjorn Helgaas]

On Tue, Jan 04, 2022 at 03:26:14PM -0400, Jason Gunthorpe wrote:
> On Tue, Jan 04, 2022 at 11:06:31AM -0600, Bjorn Helgaas wrote:
> 
> > > The existing vfio framework allows the portdrv driver to be bound
> > > to the bridge while its downstream devices are assigned to user space.
> > 
> > I.e., the existing VFIO framework allows a switch to be in the same
> > IOMMU group as the devices below it, even though the switch has a
> > kernel driver and the other devices may have userspace drivers?
> 
> Yes, this patch exists to maintain current VFIO behavior which has this
> same check.
> 
> I belive the basis for VFIO doing this is that the these devices
> cannot do DMA, so don't care about the DMA API or the group->domain,
> and do not expose MMIO memory so do not care about the P2P attack.

"These devices" means bridges, right?  Not sure why we wouldn't care
about the P2P attack.

PCIe switches use MSI or MSI-X for hotplug, PME, etc, so they do DMA
for that.  Is that not relevant here?

Is there something that *prohibits* a bridge from having
device-specific functionality including DMA?

I know some bridges have device-specific BARs for performance counters
and the like.