To: Alex Williamson <alex.williamson@redhat.com>
Cc: Bjorn Helgaas <helgaas@kernel.org>, linux-kernel@vger.kernel.org,
        linux-pci@vger.kernel.org, linux-nvme@lists.infradead.org,
        linux-rdma@vger.kernel.org, linux-nvdimm@lists.01.org,
        linux-block@vger.kernel.org, Stephen Bates <sbates@raithlin.com>,
        Christoph Hellwig <hch@lst.de>, Jens Axboe <axboe@kernel.dk>,
        Keith Busch <keith.busch@intel.com>,
        Sagi Grimberg <sagi@grimberg.me>,
        Bjorn Helgaas <bhelgaas@google.com>,
        Jason Gunthorpe <jgg@mellanox.com>,
        Max Gurtovoy <maxg@mellanox.com>,
        Dan Williams <dan.j.williams@intel.com>,
        =?UTF-8?B?SsOpcsO0bWUgR2xpc3Nl?= <jglisse@redhat.com>,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>
References: <20180104190137.7654-1-logang@deltatee.com>
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 <20180104215721.GF189897@bhelgaas-glaptop.roam.corp.google.com>
 <20180104153551.3118f71b@t450s.home>
 <20fdb5bb-0236-c093-ed53-e12664022f53@deltatee.com>
 <20180104203300.79487c98@w520.home>
From: Logan Gunthorpe <logang@deltatee.com>
Message-ID: <77ec7893-6ff5-ebdf-163d-fc4e02077cc2@deltatee.com>
Date: Fri, 5 Jan 2018 10:10:51 -0700
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Subject: Re: [PATCH 04/12] pci-p2p: Clear ACS P2P flags for all client devices
Sender: linux-kernel-owner@vger.kernel.org


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