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

[Jean Guyader <jean.guyader@xxxxxxxxxxxxx>]

On 23/11 04:41, Jan Beulich wrote:
> >>> 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.)
> 
> 

I've printed sizeof (struct domain) on boot with xen-unstable and it's
exactly a PAGE big. That mean that if I want a add a value to the struct
domain (event a pointer) I will have to make some room first.

Jean

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