[PATCH 2/4] ARM: Fix register constraints in run_in_exception_handler()

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

Right now, run_in_exception_handler() takes an input in an arbitrary register,
and clobbers BUG_FN_REG.  This causes the compiler to calculate fn in the
wrong regsiter.

Instead, use `register asm()` which is the normal way of tying register
constraints to exact registers.

Bloat-o-meter reports:

  ARM64:
    Function                                     old     new   delta
    dump_registers                               356     348      -8

  ARM32:
    ns16550_poll                                  52      48      -4
    dump_registers                               432     428      -4

The other instruction dropped in ARM64's dump_registers() is an alignment nop.

No functional change.

Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
----
CC: Stefano Stabellini <sstabellini@xxxxxxxxxx>
CC: Julien Grall <julien@xxxxxxx>
CC: Volodymyr Babchuk <Volodymyr_Babchuk@xxxxxxxx>
CC: Bertrand Marquis <bertrand.marquis@xxxxxxx>
CC: Michal Orzel <michal.orzel@xxxxxxx>
CC: Oleksii Kurochko <oleksii.kurochko@xxxxxxxxx>
---
 xen/arch/arm/include/asm/bug.h | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/xen/arch/arm/include/asm/bug.h b/xen/arch/arm/include/asm/bug.h
index cacaf014ab09..8bf71587bea1 100644
--- a/xen/arch/arm/include/asm/bug.h
+++ b/xen/arch/arm/include/asm/bug.h
@@ -59,15 +59,15 @@ struct bug_frame {
  * be called function in a fixed register.
  */
 #define  run_in_exception_handler(fn) do {                                  \
-    asm ("mov " __stringify(BUG_FN_REG) ", %0\n"                            \
-         "1:"BUG_INSTR"\n"                                                  \
+    register unsigned long _fn asm (STR(BUG_FN_REG)) = (unsigned long)(fn); \
+    asm ("1:"BUG_INSTR"\n"                                                  \
          ".pushsection .bug_frames." __stringify(BUGFRAME_run_fn) ","       \
          "             \"a\", %%progbits\n"                                 \
          "2:\n"                                                             \
          ".p2align 2\n"                                                     \
          ".long (1b - 2b)\n"                                                \
          ".long 0, 0, 0\n"                                                  \
-         ".popsection" :: "r" (fn) : __stringify(BUG_FN_REG) );             \
+         ".popsection" :: "r" (_fn) );                                      \
 } while (0)
 
 #define WARN() BUG_FRAME(BUGFRAME_warn, __LINE__, __FILE__, 0, "")
-- 
2.39.5