Re: [Xen-devel] page ref/type count overflows

[Keir Fraser <keir.fraser@xxxxxxxxxxxxx>]

On 26/01/2009 13:10, "Jan Beulich" <jbeulich@xxxxxxxxxx> wrote:

> The backend drivers in my opinion have no alternative to getting taught
> to do full error checking in order to avoid the respective DomU-induced
> problems.

Backend drivers, which get their mappings via grants, have sufficient
checking already don't they?

As for the general issue, why fudge around it when for any modern system we
can just fix it? By which I mean, x64 systems with CMPXCHG16 support
(everything but ancient Opterons?) can have a full domain pointer and a long
count_info in struct page_info, and still update both atomically.

It'd be a smaller patch, and it'd be less kludgy. Disadvantages are extra 8
bytes per page (which I think is okay, particularly to fix this nasty issue
in a clean way) and only fixes the issue for x64 (I personally don't care
about non-regressions for i386, especially when the cost of the fix in this
case would be IMO high in code complexity and ugliness).

Already there is no reason why type_info (an unsigned long) could not have a
wider count. It's just no use until count_info is widened.

What do you say to that then? :-)

 -- Keir



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