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

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Date:	Wed, 4 Dec 2013 11:33:25 +0000
From:	Stefano Stabellini <stefano.stabellini@...citrix.com>
To:	Stefano Stabellini <stefano.stabellini@...citrix.com>
CC:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Ian Campbell <Ian.Campbell@...rix.com>,
	David Vrabel <david.vrabel@...rix.com>,
	Julien Grall <julien.grall@...aro.org>,
	<linux-kernel@...r.kernel.org>, <xen-devel@...ts.xenproject.org>,
	Boris Ostrovsky <boris.ostrovsky@...cle.com>,
	Roger Pau Monne <roger.pau@...rix.com>
Subject: Re: [Xen-devel] [PATCH RFC] xen-block: correctly define structures
 in public headers

On Wed, 4 Dec 2013, Stefano Stabellini wrote:
> On Tue, 3 Dec 2013, 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
> > 
> > Since the xen-blkback has its own version of the structs there is no
> > need to change change newer and older version of it.
> > 
> > As long as that works I am OK sticking it in.
> > 
> > I think from the ARM perspective it is still in 'experimental' phase
> > so anything goes to make it work under ARM.
> 
> To be honest I am unhappy about this, but I don't want to clutter even
> more a code path already plagued by an ifdef infestation.
> 
> Even if the ARM port is experimental, I would prefer to retain
> compatibility if it was possible to do so with a couple of lines fix.
> Otherwise I would rather break ABI compatibility than introducing
> another half a dozen ifdefs.

Let me rephrase this as it can be misinterpreted as a NACK for this
patch while it is not.

I would like not to break existing ARM guests.
However we do need to fix this and I can't see a decent way to do so
retaining compatibility with the broken interface that we are currently
implementing.  Therefore I am OK with the patch.
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