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Subject: Re: [RFC] /dev/ioasid uAPI proposal
To:     "Tian, Kevin" <kevin.tian@intel.com>,
        LKML <linux-kernel@vger.kernel.org>,
        Joerg Roedel <joro@8bytes.org>,
        Jason Gunthorpe <jgg@nvidia.com>,
        Lu Baolu <baolu.lu@linux.intel.com>,
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        "iommu@lists.linux-foundation.org" <iommu@lists.linux-foundation.org>,
        "kvm@vger.kernel.org" <kvm@vger.kernel.org>,
        "Alex Williamson (alex.williamson@redhat.com)" 
        <alex.williamson@redhat.com>, Jason Wang <jasowang@redhat.com>
Cc:     Eric Auger <eric.auger@redhat.com>,
        Jonathan Corbet <corbet@lwn.net>,
        "Raj, Ashok" <ashok.raj@intel.com>,
        "Liu, Yi L" <yi.l.liu@intel.com>, "Wu, Hao" <hao.wu@intel.com>,
        "Jiang, Dave" <dave.jiang@intel.com>,
        Jacob Pan <jacob.jun.pan@linux.intel.com>,
        Jean-Philippe Brucker <jean-philippe@linaro.org>,
        David Gibson <david@gibson.dropbear.id.au>,
        Kirti Wankhede <kwankhede@nvidia.com>,
        Robin Murphy <robin.murphy@arm.com>
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From:   "Enrico Weigelt, metux IT consult" <lkml@metux.net>
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Date:   Wed, 2 Jun 2021 10:56:48 +0200
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On 27.05.21 09:58, Tian, Kevin wrote:

Hi,

> /dev/ioasid provides an unified interface for managing I/O page tables for
> devices assigned to userspace. Device passthrough frameworks (VFIO, vDPA,
> etc.) are expected to use this interface instead of creating their own logic to
> isolate untrusted device DMAs initiated by userspace.

While I'm in favour of having generic APIs for generic tasks, as well as
using FDs, I wonder whether it has to be a new and separate device.

Now applications have to use multiple APIs in lockstep. One consequence
of that is operators, as well as provisioning systems, container
infrastructures, etc, always have to consider multiple devices together.

You can't just say "give workload XY access to device /dev/foo" anymore.
Now you have to take care about scenarios like "if someone wants
/dev/foo, he also needs /dev/bar"). And if that happens multiple times
together ("/dev/foo and /dev/wurst, both require /dev/bar), leading to
scenarios like the dev nodes are bind-mounted somewhere, you need to
take care that additional devices aren't bind-mounted twice, etc ...

If I understand this correctly, /dev/ioasid is a kind of "common 
supplier" to other APIs / devices. Why can't the fd be acquired by the
consumer APIs (eg. kvm, vfio, etc) ?


--mtx

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Enrico Weigelt, metux IT consult
Free software and Linux embedded engineering
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