[Xen-devel] [PATCH v5 05/16] x86/boot: call reloc() using stdcall calling convention

[Daniel Kiper <daniel.kiper@xxxxxxxxxx>]

Current reloc() call method makes confusion and does not scale well
for more arguments. And subsequent patch adding multiboot2 protocol
support have to pass 3 arguments instead of 2. Hence, move reloc()
call to stdcall calling convention. One may argue that we should use
standard cdecl calling convention. However, stdcall is better here
than cdecl because we do not need to remove "manually" arguments from
stack in xen/arch/x86/boot/head.S assembly file.

Suggested-by: Jan Beulich <jbeulich@xxxxxxxx>
Signed-off-by: Daniel Kiper <daniel.kiper@xxxxxxxxxx>
Acked-by: Jan Beulich <jbeulich@xxxxxxxx>
---
v5 - suggestions/fixes:
   - improve commit message
     (suggested by Jan Beulich).

v4 - suggestions/fixes:
   - move to stdcall calling convention
     (suggested by Jan Beulich).

v3 - suggestions/fixes:
   - simplify assembly in xen/arch/x86/boot/reloc.c file
     (suggested by Jan Beulich),
   - reorder arguments for reloc() call from xen/arch/x86/boot/head.S
     (suggested by Jan Beulich),
   - improve commit message
     (suggested by Jan Beulich).
---
 xen/arch/x86/boot/head.S  |    3 ++-
 xen/arch/x86/boot/reloc.c |   11 ++++++-----
 2 files changed, 8 insertions(+), 6 deletions(-)

diff --git a/xen/arch/x86/boot/head.S b/xen/arch/x86/boot/head.S
index e34351c..7e5ae12 100644
--- a/xen/arch/x86/boot/head.S
+++ b/xen/arch/x86/boot/head.S
@@ -119,7 +119,8 @@ __start:
 
         /* Save the Multiboot info struct (after relocation) for later use. */
         mov     $sym_phys(cpu0_stack)+1024,%esp
-        push    %ebx
+        push    %eax                /* Boot trampoline address. */
+        push    %ebx                /* Multiboot information address. */
         call    reloc
         mov     %eax,sym_phys(multiboot_ptr)
 
diff --git a/xen/arch/x86/boot/reloc.c b/xen/arch/x86/boot/reloc.c
index 9ae42e2..28c6cea 100644
--- a/xen/arch/x86/boot/reloc.c
+++ b/xen/arch/x86/boot/reloc.c
@@ -10,15 +10,16 @@
  *    Keir Fraser <keir@xxxxxxx>
  */
 
-/* entered with %eax = BOOT_TRAMPOLINE */
+/*
+ * This entry point is entered from xen/arch/x86/boot/head.S with:
+ *   - 0x4(%esp) = MULTIBOOT_INFORMATION_ADDRESS,
+ *   - 0x8(%esp) = BOOT_TRAMPOLINE_ADDRESS.
+ */
 asm (
     "    .text                         \n"
     "    .globl _start                 \n"
     "_start:                           \n"
-    "    push %eax                     \n"
-    "    push 0x8(%esp)                \n"
-    "    call reloc                    \n"
-    "    ret  $0x4                     \n"
+    "    jmp  reloc                    \n"
     );
 
 typedef unsigned int u32;
-- 
1.7.10.4


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