[xen staging] ARM: Fix register constraints in run_in_exception_handler()

[patchbot@xxxxxxx]

commit 1f5e64755d0e25b3aadf3c1ec1a4a0f0f3665969
Author:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
AuthorDate: Fri Feb 7 22:27:53 2025 +0000
Commit:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
CommitDate: Fri May 16 22:20:21 2025 +0100

    ARM: Fix register constraints in run_in_exception_handler()
    
    For asm() statements, clobbers are disjoint from inputs and outputs.
    
    Therefore, run_in_exception_handler() specifying an input and clobbering
    BUG_FN_REG forces the compiler to set up fn in the wrong regsiter, just to
    have the code block move it into the correct register.
    
    Instead, use `register asm()` to tell the compiler which register fn really
    wants to be in.  This results in better code generation.
    
    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>
    Reviewed-by: Michal Orzel <michal.orzel@xxxxxxx>
---
 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 cacaf014ab..8bf71587be 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, "")
--
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