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

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Date:   Wed, 21 Apr 2021 13:18:07 +0000
From:   "Liu, Yi L" <yi.l.liu@...el.com>
To:     Alex Williamson <alex.williamson@...hat.com>,
        Jacob Pan <jacob.jun.pan@...ux.intel.com>
CC:     Jason Gunthorpe <jgg@...dia.com>,
        Auger Eric <eric.auger@...hat.com>,
        Jean-Philippe Brucker <jean-philippe@...aro.org>,
        "Tian, Kevin" <kevin.tian@...el.com>,
        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>
Subject: RE: [PATCH V4 05/18] iommu/ioasid: Redefine IOASID set and allocation
 APIs

Hi Alex,

> From: Alex Williamson <alex.williamson@...hat.com>
> Sent: Friday, April 16, 2021 11:46 PM
[...]
> > This is not a tactic or excuse for not working on the new /dev/ioasid
> > interface. In fact, I believe we can benefit from the lessons learned
> > while completing the existing. This will give confidence to the new
> > interface. Thoughts?
> 
> I understand a big part of Jason's argument is that we shouldn't be in
> the habit of creating duplicate interfaces, we should create one, well
> designed interfaces to share among multiple subsystems.  As new users
> have emerged, our solution needs to change to a common one rather than
> a VFIO specific one.  The IOMMU uAPI provides an abstraction, but at
> the wrong level, requiring userspace interfaces for each subsystem.
> 
> Luckily the IOMMU uAPI is not really exposed as an actual uAPI, but
> that changes if we proceed to enable the interfaces to tunnel it
> through VFIO.
> 
> The logical answer would therefore be that we don't make that
> commitment to the IOMMU uAPI if we believe now that it's fundamentally
> flawed.
> 
> Ideally this new /dev/ioasid interface, and making use of it as a VFIO
> IOMMU backend, should replace type1. 

yeah, just a double check, I think this also requires a new set of uAPIs
(e.g. new MAP/UNMAP), which means the current VFIO IOMMU type1 related ioctls
would be deprecated in future. right?

> Type1 will live on until that
> interface gets to parity, at which point we may deprecate type1, but it
> wouldn't make sense to continue to expand type1 in the same direction
> as we intend /dev/ioasid to take over in the meantime, especially if it
> means maintaining an otherwise dead uAPI.  Thanks,

understood.

Regards,
Yi Liu