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