linux-kernel - Re: [RFC PATCH v2 00/10] vfio/mdev: IOMMU aware mediated device

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Message-ID: <64e4726e-929f-0e76-6dcc-45a750bde7e9@arm.com>
Date:   Fri, 14 Sep 2018 15:40:15 +0100
From:   Jean-Philippe Brucker <jean-philippe.brucker@....com>
To:     "Tian, Kevin" <kevin.tian@...el.com>,
        Lu Baolu <baolu.lu@...ux.intel.com>,
        Joerg Roedel <joro@...tes.org>,
        David Woodhouse <dwmw2@...radead.org>,
        Alex Williamson <alex.williamson@...hat.com>,
        Kirti Wankhede <kwankhede@...dia.com>
Cc:     "Raj, Ashok" <ashok.raj@...el.com>,
        "Bie, Tiwei" <tiwei.bie@...el.com>,
        "Kumar, Sanjay K" <sanjay.k.kumar@...el.com>,
        "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "Sun, Yi Y" <yi.y.sun@...el.com>,
        "Pan, Jacob jun" <jacob.jun.pan@...el.com>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>
Subject: Re: [RFC PATCH v2 00/10] vfio/mdev: IOMMU aware mediated device

>> This example only needs to modify first-level translation, and works
>> with SMMUv3. The kernel here could be the host, in which case
>> second-level translation is disabled in the SMMU, or it could be the
>> guest, in which case second-level mappings are created by QEMU and
>> first-level translation is managed by assigning PASID tables to the guest.
> 
> the former yes applies to aux domain concept. The latter doesn't -
> you have only one second-level per device. whole PASID table managed
> by guest means you assign the whole device to guest, which is not the
> concept of aux domain here.

Right, in the latter case, the host uses a "normal" domain to assign the
whole PCI function to the guest. But the guest can still use auxiliary
domains like in my example, to sub-assign the PCI function to different
guest userspace applications.

>> So (2) would use iommu_sva_bind_device(), but (1) needs something else.
>> Aren't auxiliary domains suitable for (1)? Why limit auxiliary domain to
>> second-level or nested translation? It seems silly to use a different
>> API for first-level, since the flow in userspace and VFIO is the same as
>> your second-level case as far as MAP_DMA ioctl goes. The difference is
>> that in your case the auxiliary domain supports an additional operation
>> which binds first-level page tables. An auxiliary domain that only
>> supports first-level wouldn't support this operation, but it can still
>> implement iommu_map/unmap/etc.
> 
> Thanks for correcting me on this. You are right that aux domain shouldn't
> impose such limitation on 2nd or nested only. We define aux domain
> as a normal domain (aux takes effect only when attaching to a device),
> thus it should support all capabilities possible on a normal domain.
> 
> btw I'm not sure whether you look at my comment to patch 8/10. I
> explained the rationale why aux domain doesn't interfere with existing
> default domain usage, and in a quick thinking above example might
> not make difference. but need your confirm here. :-)

Yes sorry, I didn't have time to answer, will do it now

Thanks,
Jean