[PATCH v2] Restore memory used for IP computation

[Frediano Ziglio <frediano.ziglio@xxxxxxxxx>]

We need to write in some location but no reasons to not
trying to restore what we potentially overwrote.

Signed-off-by: Frediano Ziglio <frediano.ziglio@xxxxxxxxx>
---
 xen/arch/x86/boot/head.S | 22 ++++++++++++++--------
 1 file changed, 14 insertions(+), 8 deletions(-)
---
Changes since v1:
- Rewrite magic number field instead of some possible BIOS area.

diff --git a/xen/arch/x86/boot/head.S b/xen/arch/x86/boot/head.S
index d8ac0f0494..9b7e7b4e51 100644
--- a/xen/arch/x86/boot/head.S
+++ b/xen/arch/x86/boot/head.S
@@ -415,16 +415,19 @@ __pvh_start:
 
         /*
          * We need one push/pop to determine load address.  Use the same
-         * absolute stack address as the native path, for lack of a better
-         * alternative.
+         * stack address as the native path.
          */
-        mov     $0x1000, %esp
+        mov     %ebx, %esp
+        pop     %edx
 
         /* Calculate the load base address. */
         call    1f
 1:      pop     %esi
         sub     $sym_offs(1b), %esi
 
+        /* Restore clobbered magic field */
+        push    %edx
+
         /* Set up stack. */
         lea     STACK_SIZE - CPUINFO_sizeof + sym_esi(cpu0_stack), %esp
 
@@ -463,18 +466,21 @@ __start:
          * 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.
+         * Save and restore the magic field of start_info in ebx, and use
+         * that as the stack. See also
+         * 
https://lore.kernel.org/xen-devel/20240814195053.5564-3-jason.andryuk@xxxxxxx/
          */
-        mov     $0x1000, %esp
+        mov     %ebx, %esp
+        pop     %edx
 
         /* Calculate the load base address. */
         call    1f
 1:      pop     %esi
         sub     $sym_offs(1b), %esi
 
+        /* Restore clobbered magic field */
+        push    %edx
+
         /* Set up stack. */
         lea     STACK_SIZE - CPUINFO_sizeof + sym_esi(cpu0_stack), %esp
 
-- 
2.46.0