xen/e820: if there is no dom0_mem=, don't tweak extra_pages.

The patch "xen: use maximum reservation to limit amount of usable RAM"
(d312ae878b6aed3912e1acaaf5d0b2a9d08a4f11) breaks machines that
do not use 'dom0_mem=' argument with:

reserve RAM buffer: 000000133f2e2000 - 000000133fffffff
(XEN) mm.c:4976:d0 Global bit is set to kernel page fffff8117e
(XEN) domain_crash_sync called from entry.S
(XEN) Domain 0 (vcpu#0) crashed on cpu#0:
...

The reason being that the last E820 entry is created using the
'extra_pages' (which is based on how many pages have been freed).
The mentioned git commit sets the initial value of 'extra_pages'
using a hypercall which returns the number of pages (if dom0_mem
has been used) or -1 otherwise. If the later we return with
MAX_DOMAIN_PAGES as basis for calculation:

    return min(max_pages, MAX_DOMAIN_PAGES);

and use it:

     extra_limit = xen_get_max_pages();
     if (extra_limit >= max_pfn)
             extra_pages = extra_limit - max_pfn;
     else
             extra_pages = 0;

which means we end up with extra_pages = 128GB in PFNs (33554432)
- 8GB in PFNs (2097152, on this specific box, can be larger or smaller),
and then we add that value to the E820 making it:

  Xen: 00000000ff000000 - 0000000100000000 (reserved)
  Xen: 0000000100000000 - 000000133f2e2000 (usable)

which is clearly wrong. It should look as so:

  Xen: 00000000ff000000 - 0000000100000000 (reserved)
  Xen: 0000000100000000 - 000000027fbda000 (usable)

Naturally this problem does not present itself if dom0_mem=max:X
is used.

CC: stable@kernel.org
Signed-off-by: David Vrabel <david.vrabel@citrix.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>

diff --git a/arch/x86/xen/setup.c b/arch/x86/xen/setup.c
index ff3dfa176814ac6a7f00418bf0caf5ee45053dd3..09688eb4a899b8bd64e28b4dd4142e851dbc70bb 100644 (file)
--- a/arch/x86/xen/setup.c
+++ b/arch/x86/xen/setup.c
@@ -305,10 +305,12 @@ char * __init xen_memory_setup(void)
        sanitize_e820_map(e820.map, ARRAY_SIZE(e820.map), &e820.nr_map);
 
        extra_limit = xen_get_max_pages();
-       if (extra_limit >= max_pfn)
-               extra_pages = extra_limit - max_pfn;
-       else
-               extra_pages = 0;
+       if (max_pfn + extra_pages > extra_limit) {
+               if (extra_limit > max_pfn)
+                       extra_pages = extra_limit - max_pfn;
+               else
+                       extra_pages = 0;
+       }
 
        extra_pages += xen_return_unused_memory(xen_start_info->nr_pages, &e820);