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

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Message-ID: <868B49CE-4F0E-4A48-BE78-12149F85F1A4@raithlin.com>
Date:   Thu, 10 May 2018 17:11:14 +0000
From:   "Stephen  Bates" <sbates@...thlin.com>
To:     Logan Gunthorpe <logang@...tatee.com>,
        Christian König <christian.koenig@....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

> 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