Re: [Xen-devel] xen kernel: build failure

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 23.11.11 at 17:31, Jean Guyader <jean.guyader@xxxxxxxxx> wrote:
> On 25 October 2011 13:35, Keir Fraser <keir.xen@xxxxxxxxx> wrote:
>> On 25/10/2011 12:08, "Christoph Egger" <Christoph.Egger@xxxxxxx> wrote:
>>
>>>
>>> Hi,
>>>
>>> Compiling the xen kernel fails with:
>>>
>>> xen/arch/x86/domain.c: In function 'alloc_domain_struct'
>>> xen/arch/x86/domain.c:191: error: negative width in bit-field '<anonymous>'
>>
>> Problem is that struct domain has grown bigger than a page for some reason,
>> in your build environment.
>>
>> I can't reproduce this.
>>
>>> Removing the line
>>>
>>> BUILD_BUG_ON(sizeof(*d) > PAGE_SIZE);
>>>
>>> makes xen kernel compile again.
>>
>> But not actually work properly. We only allocate a single page for the
>> domain struct. If the struct is bigger than a page, you'll get memory
>> corruption at run time.
>>
> 
> Is there a reason for that?

Of course there is: Post-boot there shouldn't be any allocations of
order greater than zero. This is a generally advisable thing, given that
Xen can't reclaim memory arbitrarily from domains, and in particular
also necessary for tmem.

> What would be the recommended to add something
> into the struct domain now if we can't make it bigger than a page.

Put a pointer to your data (or larger parts that are already there)
instead of the data itself into struct domain, and allocate it
separately. (You may have seen a patch from Olaf Hering late
yesterday or earlier today that moved the mem_event pieces out
of there for this very reason.)


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