Re: PCI_P2PDMA_MAP_BUS_ADDR and PCIe ATS?

On Tue, 2026-08-18 at 09:43 -0300, Jason Gunthorpe wrote:
> On Tue, Aug 18, 2026 at 11:33:07AM +0200, Thomas Hellström wrote:
> > Hi!
> > 
> > I was looking at the way pci_p2pdma_distance() returns
> > PCI_P2PDMA_MAP_BUS_ADDR if devices sit beneath a local switch with
> > ACS
> > disabled or not present.
> > 
> > The client then programs the bus address directly to perform p2p
> > dma,
> > as handed out by the dma layer.
> > 
> > However if the device (p2pdma client) has IOMMU turned on and ATS
> > enabled, the device may or may not send a translation request to
> > the
> > IOMMU. Wouldn't that fail returnint an IOMMU fault if the device is
> > programmed with the bus address rather than the IOVA?
> 
> There are many goofy configurations you can setup on PCIe, something
> like this would be one of them :)
> 
> Linux only (barely) works with a few mainstream ones.
>  
> > So if ATS is enabled, shouldn't the device always be handed the
> > IOVA to
> > avoid this? 
> 
> No, Linux has no idea if the device will use ATS or not for a certain
> request. ATS is not an all or nothing feature in PCI.
> 
> If you want this kind of configuration then set the ACS flags to
> achieve it, or disable ATS in the device.
> 
> IMHO such a PCIe fabric is misconfigured. I can't see a reason why
> anyone would want to have ACS flags allowing fast path and then Linux
> setup to never supply an IOVA that can use it.

From my understanding, ATS translated requests would use the fast-path
as soon as a translation is established in the device, without the BUS
address being explicitly programmed. ACS redirect would block that. 

Admittedly untranslated requests would take the detour around the host
bridge, but a GPU device that would otherwise enable ATS by default to
offload the IOMMU TLB would outright fail with the current code with
little clue what was going on.

So wouldn't it be better to be safe than sorry and return a host bridge
map and make something usable and potentially fast out of something
currently unusable?

The only way I can see one could support the fast-path for both
untranslated and translated request would be to set up a 1:1 IOVA for
the particular mapping.

/Thomas









> 
> Jason