x86/boot: Fix the boot time relocation calculations

c/s b28044226e "x86: make Xen early boot code relocatable" introduces

    mov $sym_offs(__image_base__),%esi

to the legacy boot path.  However, this is by definition 0, which means the
boot code only functions correctly when Xen is loaded at its preferred
physical address (2M at the time of writing).

Xen does cope if loaded at an alternative physical address, if the
MULTIBOOT2_TAG_TYPE_LOAD_BASE_ADDR tag is filled in properly.  While recent
versions of Grub do fill this in appropriately, tboot does not.  (In fact,
tboot loads Xen at the preferred address, but claims a load address of 8M.)

Both Multiboot 1 and 2 specify the execution environment as being flat.  As a
result, Xen needs no help calculating the proper load address.

However, Multiboot specifies %esp as undefined.  Experimentally, using the
entry %esp is fine, but this is certainly no guarantee.  Use a temporary stack
in the first page of RAM, which is one of the safest areas to clobber.

Calculate the load address from %eip alone, and ignore
MULTIBOOT2_TAG_TYPE_LOAD_BASE_ADDR entirely.  This fixes legacy boot under
various versions of tboot.

Finally, set up the stack as soon as possible, which means the BIOS path has a
usable stack for the entirety of its duration.  Use the full available stack
size, rather than limiting to an arbitrary 1k.  One side effect is that the
MB2/EFI path continues to use the EFI stack until the trampoline is entered.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Tested-by: Sergey Dyasli <sergey.dyasli@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>

xen/arch/x86/boot/head.S

@@ -377,8 +377,26 @@ __start:
         cld
         cli
 
-        /* Load default Xen image load base address. */
-        mov     $sym_offs(__image_base__),%esi
+        /*
+         * Multiboot (both 1 and 2) specify the stack pointer as undefined
+         * when entering in BIOS circumstances.  This is unhelpful for
+         * relocatable images, where one push/pop is required to calculate
+         * images load address.
+         *
+         * On a BIOS-based system, the IVT and BDA occupy the first 5/16ths of
+         * the first page of RAM, with the rest free for use.  Use the top of
+         * this page for a temporary stack, being one of the safest locations
+         * to clobber.
+         */
+        mov     $0x1000, %esp
+
+        /* Calculate the load base address. */
+        call    1f
+1:      pop     %esi
+        sub     $sym_offs(1b), %esi
+
+        /* Set up stack. */
+        lea     STACK_SIZE + sym_esi(cpu0_stack), %esp
 
         /* Bootloaders may set multiboot{1,2}.mem_lower to a nonzero value. */
         xor     %edx,%edx
@@ -410,15 +428,6 @@ __start:
         cmp     %edi,MB2_fixed_total_size(%ebx)
         jbe     trampoline_bios_setup
 
-        /* Get Xen image load base address from Multiboot2 information. */
-        cmpl    $MULTIBOOT2_TAG_TYPE_LOAD_BASE_ADDR,MB2_tag_type(%ecx)
-        jne     .Lmb2_mem_lower
-
-        mov     MB2_load_base_addr(%ecx),%esi
-        sub     $XEN_IMG_OFFSET,%esi
-        jmp     .Lmb2_next_tag
-
-.Lmb2_mem_lower:
         /* Get mem_lower from Multiboot2 information. */
         cmpl    $MULTIBOOT2_TAG_TYPE_BASIC_MEMINFO,MB2_tag_type(%ecx)
         cmove   MB2_mem_lower(%ecx),%edx
@@ -519,9 +528,6 @@ trampoline_setup:
         mov     %esi,sym_fs(xen_phys_start)
         mov     %esi,sym_fs(trampoline_xen_phys_start)
 
-        /* Setup stack. %ss was initialized earlier. */
-        lea     1024+sym_esi(cpu0_stack),%esp
-
         mov     sym_fs(trampoline_phys),%ecx
 
         /* Get bottom-most low-memory stack address. */