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Re: [Xen-devel] [PATCH] shared_info size and padding fixes

To: Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>
Subject: Re: [Xen-devel] [PATCH] shared_info size and padding fixes
From: John Levon <levon@xxxxxxxxxxxxxxxxx>
Date: Thu, 10 Aug 2006 14:34:01 +0100
Cc: xen-devel@xxxxxxxxxxxxxxxxxxx, Jan Beulich <jbeulich@xxxxxxxxxx>
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On Thu, Aug 10, 2006 at 02:29:25PM +0100, Keir Fraser wrote:

> > No, see the previous discussion with Keir - changing the size of the
> > shared info structure is apparently not part of the ABI (hence the
> > reason for the patch and the explanation above!)
> > 
> > Personally this seems unusual at best to me, but that's the way it is,
> > and furthermore has already happened with "nmi_reason".
> Your patch looked fine, by the way, except that adding padding to every
> arch-specific portion of shared info seems unnecessary.

Imagine we want to add a new field to shared_info. It has to go after
the "arch" member, or offsets break. Once this happens, none of the
arch_shared_info's can be modified at all, as this will break the offset
of the new member.

It seems vastly better to provide some headroom *now* than to make a
mistake later.


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