Re: [PATCH v4 04/14] PCI/P2PDMA: Clear ACS P2P flags for all devices behind switches

Stephen Bates
2018-05-10T17:11:14Z

> Not to me. In the p2pdma code we specifically program DMA engines with
> the PCI bus address.

Ah yes of course. Brain fart on my part. We are not programming the P2PDMA initiator with an IOVA but with the PCI bus address...

> So regardless of whether we are using the IOMMU or
> not, the packets will be forwarded directly to the peer. If the ACS
> Redir bits are on they will be forced back to the RC by the switch and
> the transaction will fail. If we clear the ACS bits, the TLPs will go
> where we want and everything will work (but we lose the isolation of ACS).

Agreed.

> For EPs that support ATS, we should (but don't necessarily have to)
> program them with the IOVA address so they can go through the
> translation process which will allow P2P without disabling the ACS Redir
> bits -- provided the ACS direct translation bit is set. (And btw, if it
> is, then we lose the benefit of ACS protecting against malicious EPs).
> But, per above, the ATS transaction should involve only the IOVA address
> so the ACS bits not being set should not break ATS.

Well we would still have to clear some ACS bits but now we can clear only for translated addresses.

Stephen