Re: [Xen-devel] [PATCH v2 22/23] x86: make Xen early boot code relocatable

[Daniel Kiper <daniel.kiper@xxxxxxxxxx>]

On Fri, Aug 14, 2015 at 06:49:18AM -0600, Jan Beulich wrote:
> >>> On 14.08.15 at 13:52, <daniel.kiper@xxxxxxxxxx> wrote:
> > On Tue, Aug 11, 2015 at 12:48:06PM -0400, Konrad Rzeszutek Wilk wrote:
> >> On Mon, Jul 20, 2015 at 04:29:17PM +0200, Daniel Kiper wrote:
> >> > diff --git a/xen/include/asm-x86/page.h b/xen/include/asm-x86/page.h
> >> > index 87b3341..27481ac 100644
> >> > --- a/xen/include/asm-x86/page.h
> >> > +++ b/xen/include/asm-x86/page.h
> >> > @@ -283,7 +283,7 @@ extern root_pgentry_t 
> >> > idle_pg_table[ROOT_PAGETABLE_ENTRIES];
> >> >  extern l2_pgentry_t  *compat_idle_pg_table_l2;
> >> >  extern unsigned int   m2p_compat_vstart;
> >> >  extern l2_pgentry_t l2_xenmap[L2_PAGETABLE_ENTRIES],
> >> > -    l2_bootmap[L2_PAGETABLE_ENTRIES];
> >> > +    l2_bootmap[4*L2_PAGETABLE_ENTRIES];
> >>
> >> ? Why do we need to expand this to be 16kB?
> >
> > TBH, we need 8 KiB in the worst case. The worst case is when
> > next GiB starts (e.g. 1 GiB ends and 2 GiB starts) in the middle
> > of Xen image. In this situation we must hook up lower l2_bootmap
> > table with lower l3_bootmap entry, higher l2_bootmap table with
> > higher l3_bootmap entry and finally fill l2_bootmap relevant
> > tables in proper way. Sadly, this method requires more calculations.
> > To avoid that I have added 3 l2_bootmap tables and simply hook up
> > one after another with relevant l3_bootmap entries. However, if
> > you wish we can reduce number of l2_bootmap tables to two. This
> > way code will be more complicated but we will save about 8 KiB.
>
> Wouldn't it be better (simpler) to enforce, say, 16Mb alignment
> in the PE32+ header (which the EFI loader would then honor)?

Good idea but then we must enforce this for multiboot protocol (v1 and v2) too.
multiboot2 with my patches supports that solution. However, multiboot (v1) could
be a bit problematic because it means that we must set load address to 16 MiB.
Are we sure that this region is available on all machines like region starting
at 1 MiB?

Daniel

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