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

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Date:   Wed, 7 Apr 2021 16:36:54 -0300
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Jean-Philippe Brucker <jean-philippe@...aro.org>
Cc:     "Tian, Kevin" <kevin.tian@...el.com>,
        Jason Wang <jasowang@...hat.com>,
        Alex Williamson <alex.williamson@...hat.com>,
        "Raj, Ashok" <ashok.raj@...el.com>,
        Jonathan Corbet <corbet@....net>,
        Jean-Philippe Brucker <jean-philippe@...aro.com>,
        LKML <linux-kernel@...r.kernel.org>,
        "Jiang, Dave" <dave.jiang@...el.com>,
        "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
        Li Zefan <lizefan@...wei.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Tejun Heo <tj@...nel.org>,
        "cgroups@...r.kernel.org" <cgroups@...r.kernel.org>,
        "Wu, Hao" <hao.wu@...el.com>, David Woodhouse <dwmw2@...radead.org>
Subject: Re: [PATCH V4 05/18] iommu/ioasid: Redefine IOASID set and
 allocation APIs

On Wed, Apr 07, 2021 at 08:43:50PM +0200, Jean-Philippe Brucker wrote:

> * Get a container handle out of /dev/ioasid (or /dev/iommu, really.)
>   No operation available since we don't know what the device and IOMMU
>   capabilities are.
>
> * Attach the handle to a VF. With VFIO that would be
>   VFIO_GROUP_SET_CONTAINER. That causes the kernel to associate an IOMMU
>   with the handle, and decide which operations are available.

Right, this is basically the point, - the VFIO container (/dev/vfio)
and the /dev/ioasid we are talking about have a core of
similarity. ioasid is the generalized, modernized, and cross-subsystem
version of the same idea. Instead of calling it "vfio container" we
call it something that evokes the idea of controlling the iommu.

The issue is to seperate /dev/vfio generic functionality from vfio and
share it with every subsystem.

It may be that /dev/vfio and /dev/ioasid end up sharing a lot of code,
with a different IOCTL interface around it. The vfio_iommu_driver_ops
is not particularly VFIOy.

Creating /dev/ioasid may primarily start as a code reorganization
exercise.

> * With a map/unmap vIOMMU (or shadow mappings), a single translation level
>   is supported. With a nesting vIOMMU, we're populating the level-2
>   translation (some day maybe by binding the KVM page tables, but
>   currently with map/unmap ioctl).
> 
>   Single-level translation needs single VF per container. 

Really? Why?

Jason