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Date:   Thu, 13 May 2021 10:50:30 -0300
From:   Jason Gunthorpe <jgg@nvidia.com>
To:     David Gibson <david@gibson.dropbear.id.au>
Cc:     Alexey Kardashevskiy <aik@ozlabs.ru>,
        Alex Williamson <alex.williamson@redhat.com>,
        "Liu, Yi L" <yi.l.liu@intel.com>,
        Jacob Pan <jacob.jun.pan@linux.intel.com>,
        Auger Eric <eric.auger@redhat.com>,
        Jean-Philippe Brucker <jean-philippe@linaro.org>,
        "Tian, Kevin" <kevin.tian@intel.com>,
        LKML <linux-kernel@vger.kernel.org>,
        Joerg Roedel <joro@8bytes.org>,
        Lu Baolu <baolu.lu@linux.intel.com>,
        David Woodhouse <dwmw2@infradead.org>,
        "iommu@lists.linux-foundation.org" <iommu@lists.linux-foundation.org>,
        "cgroups@vger.kernel.org" <cgroups@vger.kernel.org>,
        Tejun Heo <tj@kernel.org>, Li Zefan <lizefan@huawei.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Jean-Philippe Brucker <jean-philippe@linaro.com>,
        Jonathan Corbet <corbet@lwn.net>,
        "Raj, Ashok" <ashok.raj@intel.com>, "Wu, Hao" <hao.wu@intel.com>,
        "Jiang, Dave" <dave.jiang@intel.com>
Subject: Re: [PATCH V4 05/18] iommu/ioasid: Redefine IOASID set and
 allocation APIs
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On Thu, May 13, 2021 at 04:07:07PM +1000, David Gibson wrote:
> On Wed, May 05, 2021 at 01:39:02PM -0300, Jason Gunthorpe wrote:
> > On Wed, May 05, 2021 at 02:28:53PM +1000, Alexey Kardashevskiy wrote:
> > 
> > > This is a good feature in general when let's say there is a linux supported
> > > device which has a proprietary device firmware update tool which only exists
> > > as an x86 binary and your hardware is not x86 - running qemu + vfio in full
> > > emulation would provide a way to run the tool to update a physical device.
> > 
> > That specific use case doesn't really need a vIOMMU though, does it?
> 
> Possibly not, but the mechanics needed to do vIOMMU on different host
> IOMMU aren't really different from what you need for a no-vIOMMU
> guest.  

For very simple vIOMMUs this might be true, but this new features of nesting
PASID, migration, etc, etc all make the vIOMMU complicated and
emuluating it completely alot harder.

Stuffing a vfio-pci into a guest and creating a physical map using a
single IOASID is comparably trivial.

Jason