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

[Joerg Roedel <joro@8bytes.org>]

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