x86: find a better location for the real-mode trampoline

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.  Xen was already using it
as a second source, and this patch keeps that working.  However, just
in case, let's also fetch the information from the multiboot structure,
where the boot loader should have placed it.  This way we don't
necessarily trust one of the BIOS or the multiboot loader more than
the other.

Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>

Retain the previous code, thus using the multiboot value only if it's
sane but lower than the BDA computed one. Also use the full 32-bit
mem_lower value and prefer MBI_MEMLIMITS over open coding it (requiring
a slight adjustment to multiboot.h to make its constants actually
usable in assembly code, which previously they were only meant to be).

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

--- a/xen/arch/x86/boot/head.S
+++ b/xen/arch/x86/boot/head.S
@@ -88,6 +88,20 @@ __start:
         movzwl  0x413,%eax          /* use base memory size on failure */
         shl     $10-4,%eax
 1:
+        /*
+         * Compare the value in the BDA with the information from the
+         * multiboot structure (if available) and use the smallest.
+         */
+        testb   $MBI_MEMLIMITS,(%ebx)
+        jz      2f                  /* not available? BDA value will be fine */
+        mov     4(%ebx),%edx
+        cmp     $0x100,%edx         /* is the multiboot value too small? */
+        jb      2f                  /* if so, do not use it */
+        shl     $10-4,%edx
+        cmp     %eax,%edx           /* compare with BDA value */
+        cmovb   %edx,%eax           /* and use the smaller */
+
+2:      /* Reserve 64kb for the trampoline */
         sub     $0x1000,%eax
 
         /* From arch/x86/smpboot.c: start_eip had better be page-aligned! */
--- a/xen/include/xen/multiboot.h
+++ b/xen/include/xen/multiboot.h
@@ -18,6 +18,7 @@
 #ifndef __MULTIBOOT_H__
 #define __MULTIBOOT_H__
 
+#include "const.h"
 
 /*
  * Multiboot header structure.
@@ -31,17 +32,17 @@
 /* The magic number passed by a Multiboot-compliant boot loader. */
 #define MULTIBOOT_BOOTLOADER_MAGIC     0x2BADB002
 
-#define MBI_MEMLIMITS  (1u<< 0)
-#define MBI_BOOTDEV    (1u<< 1)
-#define MBI_CMDLINE    (1u<< 2)
-#define MBI_MODULES    (1u<< 3)
-#define MBI_AOUT_SYMS  (1u<< 4)
-#define MBI_ELF_SYMS   (1u<< 5)
-#define MBI_MEMMAP     (1u<< 6)
-#define MBI_DRIVES     (1u<< 7)
-#define MBI_BIOSCONFIG (1u<< 8)
-#define MBI_LOADERNAME (1u<< 9)
-#define MBI_APM        (1u<<10)
+#define MBI_MEMLIMITS  (_AC(1,u) << 0)
+#define MBI_BOOTDEV    (_AC(1,u) << 1)
+#define MBI_CMDLINE    (_AC(1,u) << 2)
+#define MBI_MODULES    (_AC(1,u) << 3)
+#define MBI_AOUT_SYMS  (_AC(1,u) << 4)
+#define MBI_ELF_SYMS   (_AC(1,u) << 5)
+#define MBI_MEMMAP     (_AC(1,u) << 6)
+#define MBI_DRIVES     (_AC(1,u) << 7)
+#define MBI_BIOSCONFIG (_AC(1,u) << 8)
+#define MBI_LOADERNAME (_AC(1,u) << 9)
+#define MBI_APM        (_AC(1,u) << 10)
 
 #ifndef __ASSEMBLY__