[Xen-devel] [PATCH 1/2] x86/xen: do not identity map UNUSABLE regions in the machine E820

[David Vrabel <david.vrabel@xxxxxxxxxx>]

From: David Vrabel <david.vrabel@xxxxxxxxxx>

If there are UNUSABLE regions in the machine memory map, dom0 will
attempt to map them 1:1 which is not permitted by Xen and the kernel
will crash.

There isn't anything interesting in the UNUSABLE region that the dom0
kernel needs access to so we can avoid making the 1:1 mapping and
treat it as RAM.

We only do this for dom0, as we do not expect any domU to ever have a
UNUSABLE region in their pseudo-physical map.

This fixes a boot failure on hosts with tboot.

tboot marks a region in the e820 map as unusable and the dom0 kernel
would attempt to map this region and Xen does not permit unusable
regions to be mapped by guests.

  (XEN)  0000000000000000 - 0000000000060000 (usable)
  (XEN)  0000000000060000 - 0000000000068000 (reserved)
  (XEN)  0000000000068000 - 000000000009e000 (usable)
  (XEN)  0000000000100000 - 0000000000800000 (usable)
  (XEN)  0000000000800000 - 0000000000972000 (unusable)

tboot marked this region as unusable.

  (XEN)  0000000000972000 - 00000000cf200000 (usable)
  (XEN)  00000000cf200000 - 00000000cf38f000 (reserved)
  (XEN)  00000000cf38f000 - 00000000cf3ce000 (ACPI data)
  (XEN)  00000000cf3ce000 - 00000000d0000000 (reserved)
  (XEN)  00000000e0000000 - 00000000f0000000 (reserved)
  (XEN)  00000000fe000000 - 0000000100000000 (reserved)
  (XEN)  0000000100000000 - 0000000630000000 (usable)

Signed-off-by: David Vrabel <david.vrabel@xxxxxxxxxx>
---
 arch/x86/xen/setup.c |   21 +++++++++++++++++++++
 1 files changed, 21 insertions(+), 0 deletions(-)

diff --git a/arch/x86/xen/setup.c b/arch/x86/xen/setup.c
index 056d11f..5a093b7 100644
--- a/arch/x86/xen/setup.c
+++ b/arch/x86/xen/setup.c
@@ -313,6 +313,17 @@ static void xen_align_and_add_e820_region(u64 start, u64 
size, int type)
        e820_add_region(start, end - start, type);
 }
 
+void xen_ignore_unusable(struct e820entry *list, size_t map_size)
+{
+       struct e820entry *entry;
+       unsigned int i;
+
+       for (i = 0, entry = list; i < map_size; i++, entry++) {
+               if (entry->type == E820_UNUSABLE)
+                       entry->type = E820_RAM;
+       }
+}
+
 /**
  * machine_specific_memory_setup - Hook for machine specific memory setup.
  **/
@@ -353,6 +364,16 @@ char * __init xen_memory_setup(void)
        }
        BUG_ON(rc);
 
+       /*
+        * Xen won't allow a 1:1 mapping to be created to UNUSABLE
+        * regions, so if we're using the machine memory map leave the
+        * region as RAM as it is in the pseudo-physical map.
+        *
+        * UNUSABLE regions are not expected in domUs.
+        */
+       if (xen_initial_domain())
+               xen_ignore_unusable(map, memmap.nr_entries);
+
        /* Make sure the Xen-supplied memory map is well-ordered. */
        sanitize_e820_map(map, memmap.nr_entries, &memmap.nr_entries);
 
-- 
1.7.2.5


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