x86: Relocate Multiboot structures where we know they will be

accessible. GRUB2 seems to like to stick them really high sometimes
(just below 4GB).

The 32-bit C code framework that this sets up can also be used for
other stuff in future:
 * early cmdline parsing
 * relocating multiboot modules so they too are guaranteed accessible

Its interaction with normal Xen start-of-day, and with the 16-bit
assembly trampoline, needs a bit of thought.

Signed-off-by: Keir Fraser <keir.fraser@citrix.com>

4 files changed, 118 insertions, 2 deletions

diff --git a/xen/arch/x86/boot/Makefile b/xen/arch/x86/boot/Makefile
index 3340058c08..d9a7c3f963 100644
--- a/xen/arch/x86/boot/Makefile
+++ b/xen/arch/x86/boot/Makefile
@@ -1 +1,7 @@
 obj-y += head.o
+
+head.o: reloc.S
+
+# NB. BOOT_TRAMPOLINE == 0x8c000
+%.S: %.c
+	RELOC=0x8c000 $(MAKE) -f build32.mk $@
diff --git a/xen/arch/x86/boot/build32.mk b/xen/arch/x86/boot/build32.mk
new file mode 100644
index 0000000000..3223bcf4f9
--- /dev/null
+++ b/xen/arch/x86/boot/build32.mk
@@ -0,0 +1,24 @@
+XEN_ROOT=../../../..
+override XEN_TARGET_ARCH=x86_32
+CFLAGS =
+include $(XEN_ROOT)/Config.mk
+
+# Disable PIE/SSP if GCC supports them. They can break us.
+$(call cc-option-add,CFLAGS,CC,-nopie)
+$(call cc-option-add,CFLAGS,CC,-fno-stack-protector)
+$(call cc-option-add,CFLAGS,CC,-fno-stack-protector-all)
+
+CFLAGS += -Werror -fno-builtin -msoft-float
+
+%.S: %.bin
+	(od -v -t x $< | head -n -1 | \
+	sed 's/ /,0x/g' | sed 's/^[0-9]*,/ .long /') >$@
+
+%.bin: %.lnk
+	$(OBJCOPY) -O binary $< $@
+
+%.lnk: %.o
+	$(LD) $(LDFLAGS_DIRECT) -N -Ttext 0x8c000 -o $@ $<
+
+%.o: %.c
+	$(CC) $(CFLAGS) -c $< -o $@
diff --git a/xen/arch/x86/boot/head.S b/xen/arch/x86/boot/head.S
index c10aa62b2d..3360107aac 100644
--- a/xen/arch/x86/boot/head.S
+++ b/xen/arch/x86/boot/head.S
@@ -79,8 +79,11 @@ __start:
         cmp     $0x2BADB002,%eax
         jne     not_multiboot
 
-        /* Save the Multiboot info structure for later use. */
-        mov     %ebx,sym_phys(multiboot_ptr)
+        /* Save the Multiboot info struct (after relocation) for later use. */
+        mov     $sym_phys(cpu0_stack)+1024,%esp
+        push    %ebx
+        call    reloc
+        mov     %eax,sym_phys(multiboot_ptr)
 
         /* Initialize BSS (no nasty surprises!) */
         mov     $sym_phys(__bss_start),%edi
@@ -192,6 +195,9 @@ __start:
 
 #include "cmdline.S"
 
+reloc:
+#include "reloc.S"
+
         .align 16
         .globl trampoline_start, trampoline_end
 trampoline_start:
diff --git a/xen/arch/x86/boot/reloc.c b/xen/arch/x86/boot/reloc.c
new file mode 100644
index 0000000000..d0ae7dc9fc
--- /dev/null
+++ b/xen/arch/x86/boot/reloc.c
@@ -0,0 +1,80 @@
+/******************************************************************************
+ * reloc.c
+ * 
+ * 32-bit flat memory-map routines for relocating Multiboot structures
+ * and modules. This is most easily done early with paging disabled.
+ * 
+ * Copyright (c) 2009, Citrix Systems, Inc.
+ * 
+ * Authors:
+ *    Keir Fraser <keir.fraser@citrix.com>
+ */
+
+asm (
+    "    .text                         \n"
+    "    .globl _start                 \n"
+    "_start:                           \n"
+    "    mov  $_start,%edi             \n"
+    "    call 1f                       \n"
+    "1:  pop  %esi                     \n"
+    "    sub  $1b-_start,%esi          \n"
+    "    mov  $__bss_start-_start,%ecx \n"
+    "    rep  movsb                    \n"
+    "    xor  %eax,%eax                \n"
+    "    mov  $_end,%ecx               \n"
+    "    sub  %edi,%ecx                \n"
+    "    rep  stosb                    \n"
+    "    mov  $reloc,%eax              \n"
+    "    jmp  *%eax                    \n"
+    );
+
+typedef unsigned int u32;
+#include "../../../include/xen/multiboot.h"
+
+extern char _start[];
+
+static void *memcpy(void *dest, const void *src, unsigned int n)
+{
+    char *s = (char *)src, *d = dest;
+    while ( n-- )
+        *d++ = *s++;
+    return dest;
+}
+
+static void *reloc_mbi_struct(void *old, unsigned int bytes)
+{
+    static void *alloc = &_start;
+    alloc = (void *)(((unsigned long)alloc - bytes) & ~15ul);
+    return memcpy(alloc, old, bytes);
+}
+
+multiboot_info_t *reloc(multiboot_info_t *mbi_old)
+{
+    multiboot_info_t *mbi = reloc_mbi_struct(mbi_old, sizeof(*mbi));
+
+    if ( mbi->flags & MBI_CMDLINE )
+    {
+        char *cmdline_old, *p;
+        cmdline_old = (char *)mbi->cmdline;
+        for ( p = cmdline_old; *p != '\0'; p++ )
+            continue;
+        mbi->cmdline = (u32)reloc_mbi_struct(cmdline_old, p - cmdline_old + 1);
+    }
+
+    if ( mbi->flags & MBI_MODULES )
+        mbi->mods_addr = (u32)reloc_mbi_struct(
+            (module_t *)mbi->mods_addr, mbi->mods_count * sizeof(module_t));
+
+    if ( mbi->flags & MBI_MEMMAP )
+        mbi->mmap_addr = (u32)reloc_mbi_struct(
+            (memory_map_t *)mbi->mmap_addr, mbi->mmap_length);
+
+    /* Mask features we don't understand or don't relocate. */
+    mbi->flags &= (MBI_MEMLIMITS |
+                   MBI_DRIVES |
+                   MBI_CMDLINE |
+                   MBI_MODULES |
+                   MBI_MEMMAP);
+
+    return mbi;
+}