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From:   Logan Gunthorpe <logang@deltatee.com>
Message-ID: <d70cf286-f7b7-99a5-1042-022506298fed@deltatee.com>
Date:   Tue, 8 May 2018 14:19:05 -0600
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Subject: Re: [PATCH v4 04/14] PCI/P2PDMA: Clear ACS P2P flags for all devices
 behind switches
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On 08/05/18 02:13 PM, Alex Williamson wrote:
> Well, I'm a bit confused, this patch series is specifically disabling
> ACS on switches, but per the spec downstream switch ports implementing
> ACS MUST implement direct translated P2P.  So it seems the only
> potential gap here is the endpoint, which must support ATS or else
> there's nothing for direct translated P2P to do.  The switch port plays
> no part in the actual translation of the request, ATS on the endpoint
> has already cached the translation and is now attempting to use it.
> For the switch port, this only becomes a routing decision, the request
> is already translated, therefore ACS RR and EC can be ignored to
> perform "normal" (direct) routing, as if ACS were not present.  It would
> be a shame to go to all the trouble of creating this no-ACS mode to find
> out the target hardware supports ATS and should have simply used it, or
> we should have disabled the IOMMU altogether, which leaves ACS disabled.

Ah, ok, I didn't think it was the endpoint that had to implement ATS.
But in that case, for our application, we need NVMe cards and RDMA NICs
to all have ATS support and I expect that is just as unlikely. At least
none of the endpoints on my system support it. Maybe only certain GPUs
have this support.

Logan