[Xen-devel] [PATCH for-4.9] x86/boot: Fix the boot time relocation calculations

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

c/s b28044226e1 "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.

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.

Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
Tested-by: Sergey Dyasli <sergey.dyasli@xxxxxxxxxx>
---
CC: Jan Beulich <JBeulich@xxxxxxxx>
CC: Julien Grall <julien.grall@xxxxxxx>
CC: Daniel Kiper <daniel.kiper@xxxxxxxxxx>
CC: Doug Goldstein <cardoe@xxxxxxxxxx>
CC: Sergey Dyasli <sergey.dyasli@xxxxxxxxxx>

This is a regression introduced in Xen 4.9, and should therefore be fixed.
---
 xen/arch/x86/boot/head.S | 15 ++++-----------
 1 file changed, 4 insertions(+), 11 deletions(-)

diff --git a/xen/arch/x86/boot/head.S b/xen/arch/x86/boot/head.S
index 5e84e42..df28b09 100644
--- a/xen/arch/x86/boot/head.S
+++ b/xen/arch/x86/boot/head.S
@@ -377,8 +377,10 @@ __start:
         cld
         cli
 
-        /* Load default Xen image load base address. */
-        mov     $sym_offs(__image_base__),%esi
+        /* Calculate the load base address. */
+        call    1f
+1:      pop     %esi
+        sub     $sym_offs(1b), %esi
 
         /* Bootloaders may set multiboot{1,2}.mem_lower to a nonzero value. */
         xor     %edx,%edx
@@ -410,15 +412,6 @@ __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
-- 
2.1.4


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