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

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Message-ID: <20210427163954.GC1370958@nvidia.com>
Date:   Tue, 27 Apr 2021 13:39:54 -0300
From:   Jason Gunthorpe <jgg@...dia.com>
To:     David Gibson <david@...son.dropbear.id.au>
Cc:     "Tian, Kevin" <kevin.tian@...el.com>,
        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

On Tue, Apr 27, 2021 at 03:11:25PM +1000, David Gibson wrote:

> > So your proposal sort of moves the entire container/group/domain 
> > managment into /dev/ioasid and then leaves vfio only provide device
> > specific uAPI. An ioasid represents a page table (address space), thus 
> > is equivalent to the scope of VFIO container.
> 
> Right.  I don't really know how /dev/iosasid is supposed to work, and
> so far I don't see how it conceptually differs from a container.  What
> is it adding?

There are three motivating topics:
 1) /dev/vfio/vfio is only usable by VFIO and we have many interesting
    use cases now where we need the same thing usable outside VFIO
 2) /dev/vfio/vfio does not support modern stuff like PASID and
    updating to support that is going to be a big change, like adding
    multiple IOASIDs so they can be modeled as as a tree inside a
    single FD
 3) I understand there is some desire to revise the uAPI here a bit,
    ie Alex mentioned the poor mapping performance.

I would say it is not conceptually different from what VFIO calls a
container, it is just a different uAPI with the goal to be cross
subsystem.

Jason