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

[Bjorn Helgaas]

On Thu, Jan 06, 2022 at 12:12:35PM +0800, Lu Baolu wrote:
> On 1/5/22 1:06 AM, Bjorn Helgaas wrote:
> > On Tue, Jan 04, 2022 at 09:56:39AM +0800, Lu Baolu wrote:
> > > If a switch lacks ACS P2P Request Redirect, a device below the switch can
> > > bypass the IOMMU and DMA directly to other devices below the switch, so
> > > all the downstream devices must be in the same IOMMU group as the switch
> > > itself.
> > Help me think through what's going on here.  IIUC, we put devices in
> > the same IOMMU group when they can interfere with each other in any
> > way (DMA, config access, etc).
> > 
> > (We said "DMA" above, but I guess this would also apply to config
> > requests, right?)
> 
> I am not sure whether devices could interfere each other through config
> space access. The IOMMU hardware only protects and isolates DMA
> accesses, so that userspace could control DMA directly. The config
> accesses will always be intercepted by VFIO. Hence, I don't see a
> problem.

I was wondering about config accesses generated by an endpoint, e.g.,
an endpoint doing config writes to a peer or the upstream bridge.

But I think that is prohibited by spec - PCIe r5.0, sec 7.3.3, says
"Propagation of Configuration Requests from Downstream to Upstream as
well as peer-to-peer are not supported" and "Configuration Requests
are initiated only by the Host Bridge, including those passed through
the SFI CAM mechanism."

Bjorn