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Re: [PATCH v6 1/3] xen/arm: add support for run_in_exception_handler()

Hi Juergen,

On 16/01/2021 10:33, Juergen Gross wrote:
Add support to run a function in an exception handler for Arm. Do it
as on x86 via a bug_frame, but pass the function pointer via a
register (this needs to be done that way, because inline asm support
for 32-bit Arm lacks the needed functionality to reference an
arbitrary function via the bugframe).

I was going to commit the series, but then realized the commit message and comment needs some tweaking because technically GCC is supporting 'i' (I managed to get it working with -fno-pie).

So how about:

"This is needed to be done that way because GCC complains about invalid constraint when using a function pointer with "i" and PIE is enabled (default on most of GCC shipped with distro). Clang happily accepts it, so it may be a bug in GCC."

+ * Unfortunately gcc for arm 32-bit doesn't support the "i" constraint, so
+ * the easiest way to implement run_in_exception_handler() is to pass the
+ * to be called function in a fixed register.
+ */

This comment should also be updated.

I can update both while committing if you are happy with the change.

+#define  run_in_exception_handler(fn) do {                                  \
+    asm ("mov " __stringify(BUG_FN_REG) ", %0\n"                            \
+         "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) );             \
+} while (0)
  #define WARN() BUG_FRAME(BUGFRAME_warn, __LINE__, __FILE__, 0, "")
#define BUG() do { \
@@ -73,7 +91,8 @@ struct bug_frame {
  extern const struct bug_frame __start_bug_frames[],
-                              __stop_bug_frames_2[];
+                              __stop_bug_frames_2[],
+                              __stop_bug_frames_3[];
#endif /* __ARM_BUG_H__ */


Julien Grall



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