linux-kernel - RE: [PATCH V4 05/18] iommu/ioasid: Redefine IOASID set and allocation APIs

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Date:   Wed, 28 Apr 2021 07:47:56 +0000
From:   "Tian, Kevin" <kevin.tian@...el.com>
To:     Jason Gunthorpe <jgg@...dia.com>,
        David Gibson <david@...son.dropbear.id.au>
CC:     Alex Williamson <alex.williamson@...hat.com>,
        "Liu, Yi L" <yi.l.liu@...el.com>,
        Jacob Pan <jacob.jun.pan@...ux.intel.com>,
        Auger Eric <eric.auger@...hat.com>,
        Jean-Philippe Brucker <jean-philippe@...aro.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Joerg Roedel <joro@...tes.org>,
        Lu Baolu <baolu.lu@...ux.intel.com>,
        David Woodhouse <dwmw2@...radead.org>,
        "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
        "cgroups@...r.kernel.org" <cgroups@...r.kernel.org>,
        Tejun Heo <tj@...nel.org>, Li Zefan <lizefan@...wei.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Jean-Philippe Brucker <jean-philippe@...aro.com>,
        Jonathan Corbet <corbet@....net>,
        "Raj, Ashok" <ashok.raj@...el.com>, "Wu, Hao" <hao.wu@...el.com>,
        "Jiang, Dave" <dave.jiang@...el.com>,
        "Alexey Kardashevskiy" <aik@...abs.ru>
Subject: RE: [PATCH V4 05/18] iommu/ioasid: Redefine IOASID set and allocation
 APIs

> From: Jason Gunthorpe <jgg@...dia.com>
> Sent: Wednesday, April 28, 2021 1:12 AM
> 
[...]
> One option is VFIO can keep its group FD but nothing else will have
> anthing like it. However I don't much like the idea that VFIO will
> have a special and unique programming model to do that same things
> other subsystem will do. That will make it harder for userspace to
> implement.

Hi, Jason,

I have a question here. Based on discussions so far, it's clearly that the
new ioasid uAPI will differ from existing VFIO uAPI a lot, e.g. ioasid-
centric operations, no group fd, no incompatible domains, etc. Then 
I wonder how we plan to support legacy VFIO applications in this 
transition phase. Earlier you ever mentioned the desire of directly
replacing /dev/vfio/vfio with /dev/ioasid and having ioasid to present
both VFIO and new uAPI. Doesn't it imply that we have to copy the 
VFIO container/group semantics into /dev/ioasid although it's a special 
programming model only for VFIO?

Alternatively we could keep all the container/group legacy within VFIO
and having /dev/ioasid support only the new uAPI semantics. In this case
VFIO will include a shim iommu backend to connect its legacy uAPI into 
drivers/ioasid backend functions for backward compatibility. Then VFIO
will also support a new model which only uses its device uAPI to bind
to new ioasid fd w/o using any legacy container/group/iommu uAPI.
Does this sound a plan? 

Thanks
Kevin