[PATCH] xen: find a better location for the real-mode trampoline

[Paolo Bonzini <pbonzini@redhat.com>]

On some machines, the location at 0x40e does not point to the beginning
of the EBDA.  Rather, it points to the beginning of the BIOS-reserved
area of the EBDA, while the option ROMs place their data below that
segment.

For this reason, 0x413 is actually a better source than 0x40e to get
the location of the real-mode trampoline.  But it is even better to
fetch the information from the multiboot structure, where the boot
loader has placed the data for us already.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
---
 xen/arch/x86/boot/head.S | 21 ++++++++++++---------
 1 file changed, 12 insertions(+), 9 deletions(-)

diff --git a/xen/arch/x86/boot/head.S b/xen/arch/x86/boot/head.S
index 7efa155..1790462 100644
--- a/xen/arch/x86/boot/head.S
+++ b/xen/arch/x86/boot/head.S
@@ -78,16 +78,19 @@ __start:
         cmp     $0x2BADB002,%eax
         jne     not_multiboot
 
-        /* Set up trampoline segment 64k below EBDA */
-        movzwl  0x40e,%eax          /* EBDA segment */
-        cmp     $0xa000,%eax        /* sanity check (high) */
-        jae     0f
-        cmp     $0x4000,%eax        /* sanity check (low) */
-        jae     1f
-0:
-        movzwl  0x413,%eax          /* use base memory size on failure */
-        shl     $10-4,%eax
+        /* Set up trampoline segment just below end of base memory.
+         * Prefer to get this information from the multiboot
+         * structure, if available.
+         */
+        mov     4(%ebx),%eax        /* kb of low memory */
+        testb   $1,(%ebx)           /* test MBI_MEMLIMITS */
+        jnz     1f
+
+        movzwl  0x413,%eax          /* base memory size in kb */
 1:
+        shl     $10-4,%eax          /* convert to a segment number */
+
+        /* Reserve 64kb for the trampoline */
         sub     $0x1000,%eax
 
         /* From arch/x86/smpboot.c: start_eip had better be page-aligned! */
-- 
1.8.0