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From: Joerg Roedel <joro@8bytes.org>
To: Jacob Pan <jacob.jun.pan@linux.intel.com>
Cc: "Tian, Kevin" <kevin.tian@intel.com>,
	Jacob Pan <jacob.pan.linux@gmail.com>,
	Raj Ashok <ashok.raj@intel.com>, Jonathan Corbet <corbet@lwn.net>,
	Jean-Philippe Brucker <jean-philippe@linaro.com>,
	iommu@lists.linux-foundation.org,
	LKML <linux-kernel@vger.kernel.org>,
	Alex Williamson <alex.williamson@redhat.com>,
	Wu Hao <hao.wu@intel.com>, David Woodhouse <dwmw2@infradead.org>,
	Yi Sun <yi.y.sun@intel.com>
Subject: Re: [PATCH v9 3/7] iommu/uapi: Introduce enum type for PASID data format
Date: Thu, 24 Sep 2020 10:40:16 +0200	[thread overview]
Message-ID: <20200924084015.GC27174@8bytes.org> (raw)
In-Reply-To: <20200918101108.672c2f5a@jacob-builder>

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@8bytes.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
_______________________________________________
iommu mailing list
iommu@lists.linux-foundation.org
https://lists.linuxfoundation.org/mailman/listinfo/iommu

WARNING: multiple messages have this Message-ID (diff)
From: Joerg Roedel <joro@8bytes.org>
To: Jacob Pan <jacob.jun.pan@linux.intel.com>
Cc: Jacob Pan <jacob.pan.linux@gmail.com>,
	iommu@lists.linux-foundation.org,
	LKML <linux-kernel@vger.kernel.org>,
	Alex Williamson <alex.williamson@redhat.com>,
	Lu Baolu <baolu.lu@linux.intel.com>,
	David Woodhouse <dwmw2@infradead.org>,
	Jonathan Corbet <corbet@lwn.net>,
	Jean-Philippe Brucker <jean-philippe@linaro.com>,
	Eric Auger <eric.auger@redhat.com>, Yi Liu <yi.l.liu@intel.com>,
	"Tian, Kevin" <kevin.tian@intel.com>,
	Raj Ashok <ashok.raj@intel.com>, Wu Hao <hao.wu@intel.com>,
	Yi Sun <yi.y.sun@intel.com>
Subject: Re: [PATCH v9 3/7] iommu/uapi: Introduce enum type for PASID data format
Date: Thu, 24 Sep 2020 10:40:16 +0200	[thread overview]
Message-ID: <20200924084015.GC27174@8bytes.org> (raw)
In-Reply-To: <20200918101108.672c2f5a@jacob-builder>

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@8bytes.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

  parent reply	other threads:[~2020-09-24  8:40 UTC|newest]

Thread overview: 32+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-09-11 21:57 [PATCH v9 0/7] IOMMU user API enhancement Jacob Pan
2020-09-11 21:57 ` Jacob Pan
2020-09-11 21:57 ` [PATCH v9 1/7] docs: IOMMU user API Jacob Pan
2020-09-11 21:57   ` Jacob Pan
2020-09-16 18:26   ` Randy Dunlap
2020-09-16 18:26     ` Randy Dunlap
2020-09-18 19:44     ` Jacob Pan
2020-09-11 21:57 ` [PATCH v9 2/7] iommu/uapi: Add argsz for user filled data Jacob Pan
2020-09-11 21:57   ` Jacob Pan
2020-09-11 21:57 ` [PATCH v9 3/7] iommu/uapi: Introduce enum type for PASID data format Jacob Pan
2020-09-11 21:57   ` Jacob Pan
2020-09-18  9:44   ` Joerg Roedel
2020-09-18  9:44     ` Joerg Roedel
2020-09-18 17:11     ` Jacob Pan
2020-09-22 20:24       ` Jacob Pan
2020-09-22 20:24         ` Jacob Pan
2020-09-24  8:40       ` Joerg Roedel [this message]
2020-09-24  8:40         ` Joerg Roedel
2020-09-24 18:31         ` Jacob Pan
2020-09-24 18:31           ` Jacob Pan
2020-09-11 21:57 ` [PATCH v9 4/7] iommu/uapi: Use named union for user data Jacob Pan
2020-09-11 21:57   ` Jacob Pan
2020-09-11 21:57 ` [PATCH v9 5/7] iommu/uapi: Rename uapi functions Jacob Pan
2020-09-11 21:57   ` Jacob Pan
2020-09-11 21:57 ` [PATCH v9 6/7] iommu/uapi: Handle data and argsz filled by users Jacob Pan
2020-09-11 21:57   ` Jacob Pan
2020-09-11 21:57 ` [PATCH v9 7/7] iommu/vt-d: Check UAPI data processed by IOMMU core Jacob Pan
2020-09-11 21:57   ` Jacob Pan
2020-09-18 10:02 ` [PATCH v9 0/7] IOMMU user API enhancement Joerg Roedel
2020-09-18 10:02   ` Joerg Roedel
2020-09-18 17:26   ` Jacob Pan
2020-09-18 17:26     ` Jacob Pan

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