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Subject: Re: [PATCH V4 05/18] iommu/ioasid: Redefine IOASID set and allocation
 APIs
To:     Jason Gunthorpe <jgg@nvidia.com>
Cc:     "Liu, Yi L" <yi.l.liu@intel.com>,
        Jean-Philippe Brucker <jean-philippe@linaro.org>,
        "Tian, Kevin" <kevin.tian@intel.com>,
        Jacob Pan <jacob.jun.pan@linux.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>,
        Alex Williamson <alex.williamson@redhat.com>,
        Jonathan Corbet <corbet@lwn.net>,
        "Raj, Ashok" <ashok.raj@intel.com>, "Wu, Hao" <hao.wu@intel.com>,
        "Jiang, Dave" <dave.jiang@intel.com>
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 <20210415230732.GG1370958@nvidia.com>
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 <20210416140524.GI1370958@nvidia.com>
From:   Auger Eric <eric.auger@redhat.com>
Message-ID: <f71c1e37-6466-e931-be1d-b9b36d785596@redhat.com>
Date:   Fri, 16 Apr 2021 16:26:19 +0200
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Hi,
On 4/16/21 4:05 PM, Jason Gunthorpe wrote:
> On Fri, Apr 16, 2021 at 03:38:02PM +0200, Auger Eric wrote:
> 
>> The redesign requirement came pretty late in the development process.
>> The iommu user API is upstream for a while, the VFIO interfaces have
>> been submitted a long time ago and under review for a bunch of time.
>> Redesigning everything with a different API, undefined at this point, is
>> a major setback for our work and will have a large impact on the
>> introduction of features companies are looking forward, hence our
>> frustration.
> 
> I will answer both you and Jacob at once.
> 
> This is uAPI, once it is set it can never be changed.
> 
> The kernel process and philosophy is to invest heavily in uAPI
> development and review to converge on the best uAPI possible.
> 
> Many past submissions have take a long time to get this right, there
> are several high profile uAPI examples.
> 
> Do you think this case is so special, or the concerns so minor, that it
> should get to bypass all of the normal process?

That's not my intent to bypass any process. I am just trying to
understand what needs to be re-designed and for what use case.
> 
> Ask yourself, is anyone advocating for the current direction on
> technical merits alone?
> 
> Certainly the patches I last saw where completely disgusting from a
> uAPI design perspective.
> 
> It was against the development process to organize this work the way
> it was done. Merging a wack of dead code to the kernel to support a
> uAPI vision that was never clearly articulated was a big mistake.
> 
> Start from the beginning. Invest heavily in defining a high quality
> uAPI. Clearly describe the uAPI to all stake holders.
This was largely done during several confs including plumber, KVM forum,
for several years. Also API docs were shared on the ML. I don't remember
any voice was raised at those moments.

 Break up the
> implementation into patch series without dead code. Make the
> patches. Remove the dead code this group has already added.
> 
> None of this should be a surprise. The VDPA discussion and related
> "what is a mdev" over a year ago made it pretty clear VFIO is not the
> exclusive user of "IOMMU in userspace" and that places limits on what
> kind of uAPIs expansion it should experience going forward.
Maybe clear for you but most probably not for many other stakeholders.

Anyway I do not intend to further argue and I will be happy to learn
from you and work with you, Jacob, Liu and all other stakeholders to
define a better integration.

Thanks

Eric
> 
> Jason
>