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

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Date:   Thu, 26 Mar 2020 02:23:16 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     Jacob Pan <jacob.jun.pan@...ux.intel.com>
Cc:     Joerg Roedel <joro@...tes.org>,
        Alex Williamson <alex.williamson@...hat.com>,
        Lu Baolu <baolu.lu@...ux.intel.com>,
        iommu@...ts.linux-foundation.org,
        LKML <linux-kernel@...r.kernel.org>,
        David Woodhouse <dwmw2@...radead.org>,
        Jean-Philippe Brucker <jean-philippe@...aro.com>,
        "Tian, Kevin" <kevin.tian@...el.com>,
        Raj Ashok <ashok.raj@...el.com>
Subject: Re: [PATCH v2 1/3] iommu/uapi: Define uapi version and capabilities

On Wed, Mar 25, 2020 at 04:17:05PM -0700, Jacob Pan wrote:
> Having a single UAPI version to govern the user-kernel data structures
> makes compatibility check straightforward. On the contrary, supporting
> combinations of multiple versions of the data can be a nightmare to
> maintain.
> 
> This patch defines a unified UAPI version to be used for compatibility
> checks between user and kernel.

This is bullshit.  Version numbers don't scale and we've avoided them
everywhere.  You need need flags specifying specific behavior.

> +#define IOMMU_UAPI_VERSION	1
> +static inline int iommu_get_uapi_version(void)
> +{
> +	return IOMMU_UAPI_VERSION;
> +}

Also inline functions like this in UAPI headers that actually get
included by userspace programs simply don't work.