[Xen-devel] [PATCH for-xen-4.5 v4 17/18] x86/boot: use %ecx instead of %eax

[Daniel Kiper <daniel.kiper@xxxxxxxxxx>]

Use %ecx instead of %eax and do not wipe multiboot protocol identifier.
We need that info in reloc() to differentiate between multiboot (v1)
and multiboot2 protocol.

Signed-off-by: Daniel Kiper <daniel.kiper@xxxxxxxxxx>
Reviewed-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
---
v4 - suggestions/fixes:
   - improve comments
     (suggested by Andrew Cooper).
---
 xen/arch/x86/boot/head.S |   27 ++++++++++++++-------------
 1 file changed, 14 insertions(+), 13 deletions(-)

diff --git a/xen/arch/x86/boot/head.S b/xen/arch/x86/boot/head.S
index 64a0ff7..12e41f9 100644
--- a/xen/arch/x86/boot/head.S
+++ b/xen/arch/x86/boot/head.S
@@ -86,14 +86,14 @@ __start:
         jne     not_multiboot
 
         /* Set up trampoline segment 64k below EBDA */
-        movzwl  0x40e,%eax          /* EBDA segment */
-        cmp     $0xa000,%eax        /* sanity check (high) */
+        movzwl  0x40e,%ecx          /* EBDA segment */
+        cmp     $0xa000,%ecx        /* sanity check (high) */
         jae     0f
-        cmp     $0x4000,%eax        /* sanity check (low) */
+        cmp     $0x4000,%ecx        /* sanity check (low) */
         jae     1f
 0:
-        movzwl  0x413,%eax          /* use base memory size on failure */
-        shl     $10-4,%eax
+        movzwl  0x413,%ecx          /* use base memory size on failure */
+        shl     $10-4,%ecx
 1:
         /*
          * Compare the value in the BDA with the information from the
@@ -105,21 +105,22 @@ __start:
         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 */
+        cmp     %ecx,%edx           /* compare with BDA value */
+        cmovb   %edx,%ecx           /* and use the smaller */
 
 2:      /* Reserve 64kb for the trampoline */
-        sub     $0x1000,%eax
+        sub     $0x1000,%ecx
 
         /* From arch/x86/smpboot.c: start_eip had better be page-aligned! */
-        xor     %al, %al
-        shl     $4, %eax
-        mov     %eax,sym_phys(trampoline_phys)
+        xor     %cl, %cl
+        shl     $4, %ecx
+        mov     %ecx,sym_phys(trampoline_phys)
 
         /* Save the Multiboot info struct (after relocation) for later use. */
         mov     $sym_phys(cpu0_stack)+1024,%esp
-        push    %ebx
-        call    reloc
+        mov     %ecx,%eax
+        push    %ebx                /* Multiboot information address */
+        call    reloc               /* %eax contains trampoline address */
         mov     %eax,sym_phys(mbd_pa)
 
         /* Initialize BSS (no nasty surprises!) */
-- 
1.7.10.4


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