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

[Christian König]

Am 09.05.2018 um 18:45 schrieb Logan Gunthorpe:
>
> On 09/05/18 07:40 AM, Christian König wrote:
>> The key takeaway is that when any device has ATS enabled you can't
>> disable ACS without breaking it (even if you unplug and replug it).
> I don't follow how you came to this conclusion...
>   The ACS bits we'd be turning off are the ones that force TLPs addressed
> at a peer to go to the RC. However, ATS translation packets will be
> addressed to an untranslated address which a switch will not identify as
> a peer address so it should send upstream regardless the state of the
> ACS Req/Comp redirect bits.

Why would a switch not identify that as a peer address? We use the PASID 
together with ATS to identify the address space which a transaction 
should use.

If I'm not completely mistaken when you disable ACS it is perfectly 
possible that a bridge identifies a transaction as belonging to a peer 
address, which isn't what we want here.

Christian.

>
> Once the translation comes back, the ATS endpoint should send the TLP to
> the peer address with the AT packet type and it will be directed to the
> peer provided the Direct Translated bit is set (or the redirect bits are
> unset).
>
> I can't see how turning off the Req/Comp redirect bits could break
> anything except for the isolation they provide.
>
> Logan