linux-kernel - Re: [PATCH v2 1/3] iommu/uapi: Define uapi version and capabilities

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Message-ID: <20200414081500.GB7315@infradead.org>
Date:   Tue, 14 Apr 2020 01:15:00 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     Alex Williamson <alex.williamson@...hat.com>
Cc:     Jacob Pan <jacob.jun.pan@...ux.intel.com>,
        "Tian, Kevin" <kevin.tian@...el.com>,
        Christoph Hellwig <hch@...radead.org>,
        Joerg Roedel <joro@...tes.org>,
        Lu Baolu <baolu.lu@...ux.intel.com>,
        "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
        LKML <linux-kernel@...r.kernel.org>,
        David Woodhouse <dwmw2@...radead.org>,
        Jean-Philippe Brucker <jean-philippe@...aro.com>,
        "Raj, Ashok" <ashok.raj@...el.com>
Subject: Re: [PATCH v2 1/3] iommu/uapi: Define uapi version and capabilities

On Mon, Apr 13, 2020 at 04:21:29PM -0600, Alex Williamson wrote:
> Is the objection to a global version or to any version fields?  I don't
> really understand the global version, I'd think a mechanism to check
> extensions plus a per structure flags/version would be preferred.  The
> former should resolve how userspace can test support for features
> requiring multiple interfaces.  A global version also implies that
> we're only ever adding features and never removing.  For example,
> feature Foo is added in version 4, but it's replaced by feature Bar in
> version 5, now userspace can't simply test version >= 4 must include
> feature Foo.

The objection is to versions vs the much more sensible struct size +
capability flags.  Making it global just increases the problems with
a version for all of the above reasons.

> It seems to me that version and flags can also be complimentary, for
> example a field might be defined by a version but a flag could indicate
> if it's implemented.  With only the flag, we'd infer the field from the
> flag, with only the version we'd need to assume the field is always
> implemented.  So I have a hard time making a blanket statement that all
> versions fields should be avoided.

s/version/struct size/, but otherwise agreed.