Re: [PATCH v3 1/1] PCI: Add translated request only flag for pci_enable_pasid()

[Bjorn Helgaas]

[Joerg, you may be able to answer this.  Patch under discussion is:
https://lore.kernel.org/r/20230114073420.759989-1-baolu.lu@xxxxxxxxxxxxxxx]

On Thu, Feb 02, 2023 at 11:08:25AM +0800, Baolu Lu wrote:
> ...

> ACS is unnecessary for the devices that only use translated memory request
> for PASID. All translated addresses are granted by the Linux kernel which
> ensures that such addresses will never be in a P2P address, i.e., it's not
> contained in any bridge aperture, will *always* be routed toward the RC.

Re 201007ef707a ("PCI: Enable PASID only when ACS RR & UF enabled on
upstream path"), does that commit actually *fix* anything?  I wonder
whether we could revert it completely.

The intent of 201007ef707a is to use ACS to prevent misrouting, which
would happen if a TLP contained an address that *looked* like a PCI
bus address, i.e., it was inside a host bridge aperture, but was
*intended* to reach an IOMMU or main memory directly.

201007ef707a only affects pci_enable_pasid(), so I think we already
avoid this misrouting by restricting DMA address allocation for both
non-IOMMU scenarios and non-PASID IOMMU scenarios.

So what about PASID mappings, e.g., consider a mapping of (Requester
ID, PASID, Untranslated Address) -> Translated Address?  If either the
Untranslated Address or the Translated Address looks like a PCI bus
address, a Memory Request or Translation Request could be misrouted.

Does that actually happen?  I assume it does not happen for Translated
Addresses because that's basically the non-IOMMU case, and we don't
need ACS to prevent misrouting there.

Do IOMMUs allocate (PASID, Untranslated Addresses) that look like PCI
bus addresses?

Bjorn