Re: [PATCH] reserve end-of-conventional-memory to 1MB on 32-bit

Alexander van Heukelum
2008-02-29T18:38:57Z

On Fri, 29 Feb 2008 17:14:07 +0000, "Mark McLoughlin"
<markmc@redhat.com> said:
> On Fri, 2008-02-29 at 12:49 +0100, Alexander van Heukelum wrote:
> > This patch adds explicit detection of the EBDA and reservation
> > of the rom and adapter address space 0xa0000-0x100000 to the
> > i386 kernels. It uses reserve_bootmem instead of reserve_early,
> > because reserve_early is not yet available on i386.
> >
> > Before this patch, the EBDA size was hardcoded as 4Kb. Also, the
> > reservation of the adapter range was done by modifying the e820
> > map which is now not necessary any longer, and the code is removed
> > from copy_e820_map.
> >
> > The changes in e820_64.c are only a change in the comment above
> > copy_e820_map, and some changes of the types of local variables
> > in that function such that the 32 and 64 bit versions become equal.
> >
> > Signed-off-by: Alexander van Heukelum <heukelum@fastmail.fm>
> >
> > ---
> >
> > On Thu, Feb 28, 2008 at 09:09:56PM +0000, Ian Campbell wrote:
> > > On Thu, 2008-02-28 at 14:13 +0100, Alexander van Heukelum wrote:
> > > > The 32-bit code still uses reserve_bootmem, so this is not really
> > > > a unification with the 64-bit version of the ebda reservation code,
> > > > but at least it provides the same detection logic and reserves the
> > > > same areas.
> > > >
> > > > This does not crash immediately on qemu. No further testing was
> > > > done! Otherwise:
> > >
> > > I haven't tested extensively either but it does seem to solve the
> > > problem for Xen.
> > >
> > > Thanks!
> > > Ian
> >
> > Thank you!
> >
> > Ingo,
> >
> > I think this is ready for -x86#testing.
> > It boots to a small userspace in qemu (i386).
> > If I should separate the cleanups, let me know.
>
> I haven't investigated in any detail, but with 2.6.25-rc3 and your
> patch I'm seeing a Xen guest hit this BUG:
>
> void __init smp_alloc_memory(void)
> {
> trampoline_base = alloc_bootmem_low_pages(PAGE_SIZE);
> /*
> * Has to be in very low memory so we can execute
> * real-mode AP code.
> */
> if (__pa(trampoline_base) >= 0x9F000)
> BUG();
> }
>
> Stack looks like:
>
> [<c137ef97>] smp_alloc_memory+0x25 <--
> [<c137ef97>] smp_alloc_memory+0x25
> [<c137a500>] setup_arch+0x28e
> [<c13735f7>] start_kernel+0x7a
> [<c1379240>] xen_start_kernel+0x300

Ouch.

My first guess is that the BIOS data area is completely non-existent for
Xen.
Is it guaranteed that the memory is zeroed out on boot? In that case we
can
special-case it easily:

change:
/* Paranoia: should never happen, but... */
if (lowmem >= 0x100000)
lowmem = 0xa0000;

into:
/* Strange case, like Xen ;) */
if (lowmem == 0 || lowmem >= 0x100000)
lowmem = 0x9f000;

Can you test that?

Greetings,
Alexander
--
Alexander van Heukelum
heukelum@fastmail.fm

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