Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Date:   Fri, 7 Feb 2020 09:47:32 +0100
From:   Jean-Philippe Brucker <jean-philippe@linaro.org>
To:     Jacob Pan <jacob.jun.pan@linux.intel.com>
Cc:     Alex Williamson <alex.williamson@redhat.com>,
        iommu@lists.linux-foundation.org,
        LKML <linux-kernel@vger.kernel.org>,
        Lu Baolu <baolu.lu@linux.intel.com>,
        Joerg Roedel <joro@8bytes.org>,
        David Woodhouse <dwmw2@infradead.org>,
        Yi Liu <yi.l.liu@intel.com>,
        "Tian, Kevin" <kevin.tian@intel.com>,
        Raj Ashok <ashok.raj@intel.com>,
        Christoph Hellwig <hch@infradead.org>,
        Jean-Philippe Brucker <jean-philippe@linaro.com>,
        Jonathan Cameron <jic23@kernel.org>,
        Eric Auger <eric.auger@redhat.com>
Subject: Re: [PATCH 3/3] iommu/uapi: Add helper function for size lookup
Message-ID: <20200207084732.GA1994440@myrica>
References: <1580277724-66994-1-git-send-email-jacob.jun.pan@linux.intel.com>
 <1580277724-66994-4-git-send-email-jacob.jun.pan@linux.intel.com>
 <20200129144046.3f91e4c1@w520.home>
 <20200129151951.2e354e37@w520.home>
 <20200131155125.53475a72@jacob-builder>
 <20200203112708.14174ce2@w520.home>
 <20200203124143.05061d1e@jacob-builder>
 <20200203141236.4e2d7a74@w520.home>
 <20200203144102.643f9684@jacob-builder>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20200203144102.643f9684@jacob-builder>
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

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