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

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Date:   Thu, 10 May 2018 14:52:46 +0200
From:   Christian König <christian.koenig@....com>
To:     Logan Gunthorpe <logang@...tatee.com>,
        Stephen Bates <sbates@...thlin.com>,
        Jerome Glisse <jglisse@...hat.com>
Cc:     Alex Williamson <alex.williamson@...hat.com>,
        Bjorn Helgaas <helgaas@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
        "linux-nvme@...ts.infradead.org" <linux-nvme@...ts.infradead.org>,
        "linux-rdma@...r.kernel.org" <linux-rdma@...r.kernel.org>,
        "linux-nvdimm@...ts.01.org" <linux-nvdimm@...ts.01.org>,
        "linux-block@...r.kernel.org" <linux-block@...r.kernel.org>,
        Christoph Hellwig <hch@....de>, Jens Axboe <axboe@...nel.dk>,
        Keith Busch <keith.busch@...el.com>,
        Sagi Grimberg <sagi@...mberg.me>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Jason Gunthorpe <jgg@...lanox.com>,
        Max Gurtovoy <maxg@...lanox.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>
Subject: Re: [PATCH v4 04/14] PCI/P2PDMA: Clear ACS P2P flags for all devices
 behind switches

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