Re: [Xen-devel] [PATCH RFC] xen-block: correctly define structures in public headers

[Ian Campbell <Ian.Campbell@citrix.com>]

On Tue, 2013-12-03 at 15:11 -0500, Konrad Rzeszutek Wilk wrote:
> > > If Konrad and Boris agree that breaking the kernel's ABI in this way is
> > > acceptable in this specific case, I'll defer to them.
> > 
> > My opinion as Xen on ARM hypervisor maintainer is that this is the right
> > thing to do in this case.
> 
> Heh. If somebody can guarantee me that (by testing the right variants and
> mentioning this in the git commit) that this does not break x86, then
> I am fine.
> 
> And by 'break x86' I mean that this combination works:
>  32-bit domU on 64-bit dom0
>  64-bit domU on 32-bit dom0
> 
> And perhaps also the obvious:
>  64-bit domU on 64-bit dom0
>  32-bit domU on 32-bit dom0

One way to test this is with gdb on a vmlinux for each arch with
CONFIG_DEBUG_INFO=y. For each MEMBER of each interesting STRUCT:
        (gdb) print &((struct STRUCT *)0)->MEMBER
(this is effectively an open coded offsetof)

This could probably even be semi automated by producing a script to feed
to gdb which run through all of the options and diffing the result.

If I could have the moon on a stick I would have a tool such as this
running against the canonical Xen headers, to catch breakage as it is
introduced upstream and a tool which could run against an arbitrary ELF
binary to validate it against the upstream results.
tools/include/xen-foreign/mkchecker.py goes some way towards that but
isn't really extensible to the extent we would need/want.

While I'm asking for unicorns a gcc __attribute__((warn_on_holes)) which
could be applied to a struct to enforce the need for explicit padding
would probably be incredibly useful for this of thing.

> Since the xen-blkback has its own version of the structs there is no
> need to change change newer and older version of it.

Someone should check that these are producing the right interface on ARM
though!

> As long as that works I am OK sticking it in.

Thanks.

> I think from the ARM perspective it is still in 'experimental' phase
> so anything goes to make it work under ARM.

Ian.