Re: [PATCH] IOMMU/x86: per-domain control structure is not HVM-specific

[George Dunlap <george.dunlap@citrix.com>]

On 03/05/16 11:18, Andrew Cooper wrote:
> On 03/05/16 10:59, Jan Beulich wrote:
>>>>> On 03.05.16 at 11:36, <andrew.cooper3@citrix.com> wrote:
>>> On 02/05/16 13:55, Jan Beulich wrote:
>>>> ... and hence should not live in the HVM part of the PV/HVM union. In
>>>> fact it's not even architecture specific (there already is a per-arch
>>>> extension type to it), so it gets moved out right to common struct
>>>> domain.
>>>>
>>>> Signed-off-by: Jan Beulich <jbeulich@suse.com>
>>> Yikes - this really does need fixing ASAP.
>> The more interesting question here is how to deal with the stable
>> trees: The patch, due to its size, doesn't really lend itself to
>> backporting. Yet the alternative (mentioned in a remark in the
>> original submission) would be pretty clumsy too.
>>
>> Opinions?
>
> Once the structure layout has moved, the compiler (and/or grep) will
> highlight all the other uses.
>
> In this instance, I would be tempted to suggest that we just do a custom
> backport for each stable tree.
>
> The only thing a BUILD_BUG_ON() will do is force someone else to
> backport this patch if it actually triggers.

It will, however, avoid the code churn to the stable trees if (as 
expected) the BUILD_BUG_ON() does *not* trigger.

I think both are reasonable approaches.  I find it hard to imagine how 
new bugs could be introduced by moving the structure; the main risk 
would be that it exposes latent bugs.  But there's an equal probability 
that there are latent bugs with the current layout.

Doing a more complete backport may make future bug fixes which touch 
this code easier to backport as well.

  -George

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