linux-kernel - Re: [PATCH 3/3] iommu/uapi: Add helper function for size lookup

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Date:   Fri, 7 Feb 2020 09:47:32 +0100
From:   Jean-Philippe Brucker <jean-philippe@...aro.org>
To:     Jacob Pan <jacob.jun.pan@...ux.intel.com>
Cc:     Alex Williamson <alex.williamson@...hat.com>,
        iommu@...ts.linux-foundation.org,
        LKML <linux-kernel@...r.kernel.org>,
        Lu Baolu <baolu.lu@...ux.intel.com>,
        Joerg Roedel <joro@...tes.org>,
        David Woodhouse <dwmw2@...radead.org>,
        Yi Liu <yi.l.liu@...el.com>,
        "Tian, Kevin" <kevin.tian@...el.com>,
        Raj Ashok <ashok.raj@...el.com>,
        Christoph Hellwig <hch@...radead.org>,
        Jean-Philippe Brucker <jean-philippe@...aro.com>,
        Jonathan Cameron <jic23@...nel.org>,
        Eric Auger <eric.auger@...hat.com>
Subject: Re: [PATCH 3/3] iommu/uapi: Add helper function for size lookup

On Mon, Feb 03, 2020 at 02:41:02PM -0800, Jacob Pan wrote:
> Yeah, that would work as well. I just feel IOMMU UAPI is unlikely to get
> updated frequently, should be much less than adding new capabilities.
> I think argsz could be viewed as the version field set by the
> user, minsz is what kernel current code supports.
> 
> So let me summarize the options we have
> 1. Disallow adding new members to each structure other than reuse
> padding bits or adding union members at the end.
> 2. Allow extension of the structures beyond union, but union size has
> to be fixed with reserved spaces
> 3. Adopt VFIO argsz scheme, I don't think we need version for each
> struct anymore. argsz implies the version that user is using assuming
> UAPI data is extension only.
> 
> Jean, Eric, any comments? My preference is #1. In the apocalyptic event
> when we run out of padding, perhaps we can introduce a new API_v2 :)

I agree, new extensions will most likely want to extend the vendor
specific structures at the end rather than introduce new common fields, so
I prefer #1 which avoids fixing the union size.

Thanks,
Jean