Re: [Xen-devel] Crash on boot with 2.6.37-rc8-git3

[Ian Campbell <Ian.Campbell@xxxxxxxxxxxxx>]

On Tue, 2011-01-25 at 13:31 +0000, Stefano Stabellini wrote:
> On Tue, 25 Jan 2011, Ian Campbell wrote:
> > > It turns out that it is me having the same issue you have and not the
> > > other way around :)
> > > 
> > > Your patch (in addition to my previous patch) makes my testbox boot, no
> > > matter what dom0_mem parameter I choose.
> > > 
> > > Appended is a version of the patch that doesn't assume that the memory
> > > region starts on a page boundary.
> > > 
> > > ---
> > > 
> > > diff --git a/arch/x86/xen/setup.c b/arch/x86/xen/setup.c
> > > index b5a7f92..a3d28a1 100644
> > > --- a/arch/x86/xen/setup.c
> > > +++ b/arch/x86/xen/setup.c
> > > @@ -179,7 +179,10 @@ char * __init xen_memory_setup(void)
> > >   e820.nr_map = 0;
> > >   xen_extra_mem_start = mem_end;
> > >   for (i = 0; i < memmap.nr_entries; i++) {
> > > -         unsigned long long end = map[i].addr + map[i].size;
> > > +         unsigned long long end;
> > > +         if (map[i].type == E820_RAM)
> > > +                 map[i].size -= (map[i].size + map[i].addr) % PAGE_SIZE;
> > 
> > The more normal idiom to round down to a page boundary in the kernel is:
> >     map[i].size &= ~(PAGE_SIZE-1);
> > 
> > Do you also need to page align map[i].addr upwards for maximum safety?
> > 
> 
> unless I am very confused
> 
> map[i].size -= (map[i].size + map[i].addr) % PAGE_SIZE
> 
> is not the same as:
> 
> as map[i].size &= ~(PAGE_SIZE-1): 
> 
> because it also takes into account the possibility that map[i].addr is
> not page aligned.

Oh yes, I didn't notice that aspect of it.

>  It doesn't move map[i].addr upward but still makes sure that
> the region ends at a page boundary anyway.

Which returns to my second question ;-) Why do we not need to align addr
too?

Ian.


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