[PATCH v2] xen/arch/x86: Fix early boot command line parsing

[Daniel Kiper <daniel.kiper@oracle.com>]

There is no reliable way to encode NUL character as a character so encode
it as a number. Read: http://sourceware.org/binutils/docs/as/Characters.html.
Octal and hex encoding do not work on at least one system (GNU assembler
version 2.22 (x86_64-linux-gnu) using BFD version (GNU Binutils for Debian) 2.22).
Without this fix e.g. no-real-mode option at the end of xen.gz command line
is not detected.

v2 - suggestions/fixes:
   - add relevant comment to assembly file
     (suggested by Andrew Cooper),
   - leave encoding of other characters as is for readability
     (suggested by Jan Beulich).

Signed-off-by: Daniel Kiper <daniel.kiper@oracle.com>
---
 xen/arch/x86/boot/cmdline.S |   10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/xen/arch/x86/boot/cmdline.S b/xen/arch/x86/boot/cmdline.S
index 05ffb94..b83975e 100644
--- a/xen/arch/x86/boot/cmdline.S
+++ b/xen/arch/x86/boot/cmdline.S
@@ -138,7 +138,15 @@
         call    .Lstrlen
         add     $4,%esp
         xadd    %eax,%ebx
-        cmpb    $'\0',(%ebx)
+        /*
+         * There is no reliable way to encode NUL character as a character so encode
+         * it as a number. Read: http://sourceware.org/binutils/docs/as/Characters.html.
+         * Octal and hex encoding do not work on at least one system (GNU assembler
+         * version 2.22 (x86_64-linux-gnu) using BFD version (GNU Binutils for Debian) 2.22).
+         * Without this fix e.g. no-real-mode option at the end of xen.gz command line
+         * is not detected.
+         */
+        cmpb    $0,(%ebx)
         je      3f
         cmpb    $' ',(%ebx)
         je      3f
-- 
1.7.10.4