linux-kernel - Re: [PATCH v9 3/7] iommu/uapi: Introduce enum type for PASID data format

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Message-ID: <20200924084015.GC27174@8bytes.org>
Date:   Thu, 24 Sep 2020 10:40:16 +0200
From:   Joerg Roedel <joro@...tes.org>
To:     Jacob Pan <jacob.jun.pan@...ux.intel.com>
Cc:     Jacob Pan <jacob.pan.linux@...il.com>,
        iommu@...ts.linux-foundation.org,
        LKML <linux-kernel@...r.kernel.org>,
        Alex Williamson <alex.williamson@...hat.com>,
        Lu Baolu <baolu.lu@...ux.intel.com>,
        David Woodhouse <dwmw2@...radead.org>,
        Jonathan Corbet <corbet@....net>,
        Jean-Philippe Brucker <jean-philippe@...aro.com>,
        Eric Auger <eric.auger@...hat.com>,
        Yi Liu <yi.l.liu@...el.com>,
        "Tian, Kevin" <kevin.tian@...el.com>,
        Raj Ashok <ashok.raj@...el.com>, Wu Hao <hao.wu@...el.com>,
        Yi Sun <yi.y.sun@...el.com>
Subject: Re: [PATCH v9 3/7] iommu/uapi: Introduce enum type for PASID data
 format

Hi Jacob,

On Fri, Sep 18, 2020 at 10:11:08AM -0700, Jacob Pan wrote:
> On Fri, 18 Sep 2020 11:44:50 +0200, Joerg Roedel <joro@...tes.org> wrote:
> 
> > On Fri, Sep 11, 2020 at 02:57:52PM -0700, Jacob Pan wrote:
> > > There can be multiple vendor-specific PASID data formats used in UAPI
> > > structures. This patch adds enum type with a last entry which makes
> > > range checking much easier.  
> > 
> > But it also makes it much easier to screw up the numbers (which are ABI)
> > by inserting a new value into the middle. I prefer defines here, or
> > alternativly BUILD_BUG_ON() checks for the numbers.
> > 
> I am not following, the purpose of IOMMU_PASID_FORMAT_LAST *is* for
> preparing the future insertion of new value into the middle.
> The checking against IOMMU_PASID_FORMAT_LAST is to protect ABI
> compatibility by making sure that out of range format are rejected in all
> versions of the ABI.

But with the enum you could have:

enum {
	VTD_FOO,
	SMMU_FOO,
	LAST,
};

which makes VTD_FOO==0 and SMMU_FOO==1, and when in the next version
someone adds:

enum {
	VTD_FOO,
	VTD_BAR,
	SMMU_FOO,
	LAST,
};

then SMMU_FOO will become 2 and break ABI. So I'd like to have this
checked somewhere.

Regards,

	Joerg