linux-kernel - Re: [PATCH 04/12] pci-p2p: Clear ACS P2P flags for all client devices

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Date:   Fri, 5 Jan 2018 10:10:51 -0700
From:   Logan Gunthorpe <logang@...tatee.com>
To:     Alex Williamson <alex.williamson@...hat.com>
Cc:     Bjorn Helgaas <helgaas@...nel.org>, linux-kernel@...r.kernel.org,
        linux-pci@...r.kernel.org, linux-nvme@...ts.infradead.org,
        linux-rdma@...r.kernel.org, linux-nvdimm@...ts.01.org,
        linux-block@...r.kernel.org, Stephen Bates <sbates@...thlin.com>,
        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>,
        Jérôme Glisse <jglisse@...hat.com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>
Subject: Re: [PATCH 04/12] pci-p2p: Clear ACS P2P flags for all client devices



On 04/01/18 08:33 PM, Alex Williamson wrote:
> That's exactly what IOMMU groups represent, the smallest set of devices
> which have DMA isolation from other devices.  By poking this hole, the
> IOMMU group is invalid.  We cannot turn off ACS only for a specific
> device, in order to enable p2p it needs to be disabled at every
> downstream port between the devices where we want to enable p2p.
> Depending on the topology, that could mean we're also enabling p2p for
> unrelated devices.  Those unrelated devices might be in active use and
> the p2p IOVAs now have a different destination which is no longer IOMMU
> translated.

Oh, so IOMMU groups are created based on the existing hierarchy at boot 
time and not based on the user's needs for isolation?

Logan