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

[Jean Guyader <jean.guyader@xxxxxxxxxxxxx>]

On 28/11 11:59, Jean Guyader wrote:
> 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.
> 

I can see two candidates, that could be turned into pointers in arch_domain
for x86:

    /* nestedhvm: translate l2 guest physical to host physical */
    struct p2m_domain *nested_p2m[MAX_NESTEDP2M];
    mm_lock_t nested_p2m_lock;
or
    struct PITState vpit;

Let me know which one I should pick. Maybe I should modify both to save
some space for later.

Jean

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