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

[Stephen Bates]

> 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