[PATCH v4 1/3] xen/arm: add support for run_in_exception_handler()

[Juergen Gross <jgross@xxxxxxxx>]

Add support to run a function in an exception handler for Arm. Do it
the same way as on x86 via a bug_frame.

Unfortunately inline assembly on Arm seems to be less capable than on
x86, leading to functions called via run_in_exception_handler() having
to be globally visible.

Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
---
V4:
- new patch

I have verified the created bugframe is correct by inspecting the
created binary.

Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
---
 xen/arch/arm/traps.c       | 10 +++++++++-
 xen/drivers/char/ns16550.c |  3 ++-
 xen/include/asm-arm/bug.h  | 32 +++++++++++++++++++++-----------
 3 files changed, 32 insertions(+), 13 deletions(-)

diff --git a/xen/arch/arm/traps.c b/xen/arch/arm/traps.c
index 22bd1bd4c6..6e677affe2 100644
--- a/xen/arch/arm/traps.c
+++ b/xen/arch/arm/traps.c
@@ -1236,8 +1236,16 @@ int do_bug_frame(const struct cpu_user_regs *regs, 
vaddr_t pc)
     if ( !bug )
         return -ENOENT;
 
+    if ( id == BUGFRAME_run_fn )
+    {
+        void (*fn)(const struct cpu_user_regs *) = bug_ptr(bug);
+
+        fn(regs);
+        return 0;
+    }
+
     /* WARN, BUG or ASSERT: decode the filename pointer and line number. */
-    filename = bug_file(bug);
+    filename = bug_ptr(bug);
     if ( !is_kernel(filename) )
         return -EINVAL;
     fixup = strlen(filename);
diff --git a/xen/drivers/char/ns16550.c b/xen/drivers/char/ns16550.c
index 9235d854fe..dd6500acc8 100644
--- a/xen/drivers/char/ns16550.c
+++ b/xen/drivers/char/ns16550.c
@@ -192,7 +192,8 @@ static void ns16550_interrupt(
 /* Safe: ns16550_poll() runs as softirq so not reentrant on a given CPU. */
 static DEFINE_PER_CPU(struct serial_port *, poll_port);
 
-static void __ns16550_poll(struct cpu_user_regs *regs)
+/* run_in_exception_handler() on Arm requires globally visible symbol. */
+void __ns16550_poll(struct cpu_user_regs *regs)
 {
     struct serial_port *port = this_cpu(poll_port);
     struct ns16550 *uart = port->uart;
diff --git a/xen/include/asm-arm/bug.h b/xen/include/asm-arm/bug.h
index 36c803357c..a7da2c306f 100644
--- a/xen/include/asm-arm/bug.h
+++ b/xen/include/asm-arm/bug.h
@@ -15,34 +15,38 @@
 
 struct bug_frame {
     signed int loc_disp;    /* Relative address to the bug address */
-    signed int file_disp;   /* Relative address to the filename */
+    signed int ptr_disp;    /* Relative address to the filename or function */
     signed int msg_disp;    /* Relative address to the predicate (for ASSERT) 
*/
     uint16_t line;          /* Line number */
     uint32_t pad0:16;       /* Padding for 8-bytes align */
 };
 
 #define bug_loc(b) ((const void *)(b) + (b)->loc_disp)
-#define bug_file(b) ((const void *)(b) + (b)->file_disp);
+#define bug_ptr(b) ((const void *)(b) + (b)->ptr_disp);
 #define bug_line(b) ((b)->line)
 #define bug_msg(b) ((const char *)(b) + (b)->msg_disp)
 
-#define BUGFRAME_warn   0
-#define BUGFRAME_bug    1
-#define BUGFRAME_assert 2
+#define BUGFRAME_run_fn 0
+#define BUGFRAME_warn   1
+#define BUGFRAME_bug    2
+#define BUGFRAME_assert 3
 
-#define BUGFRAME_NR     3
+#define BUGFRAME_NR     4
 
 /* Many versions of GCC doesn't support the asm %c parameter which would
  * be preferable to this unpleasantness. We use mergeable string
  * sections to avoid multiple copies of the string appearing in the
  * Xen image.
  */
-#define BUG_FRAME(type, line, file, has_msg, msg) do {                      \
+#define BUG_FRAME(type, line, ptr, ptr_is_file, has_msg, msg) do {          \
     BUILD_BUG_ON((line) >> 16);                                             \
     BUILD_BUG_ON((type) >= BUGFRAME_NR);                                    \
     asm ("1:"BUG_INSTR"\n"                                                  \
          ".pushsection .rodata.str, \"aMS\", %progbits, 1\n"                \
-         "2:\t.asciz " __stringify(file) "\n"                               \
+         "2:\n"                                                             \
+         ".if " #ptr_is_file "\n"                                           \
+         "\t.asciz " __stringify(ptr) "\n"                                  \
+         ".endif\n"                                                         \
          "3:\n"                                                             \
          ".if " #has_msg "\n"                                               \
          "\t.asciz " #msg "\n"                                              \
@@ -52,21 +56,27 @@ struct bug_frame {
          "4:\n"                                                             \
          ".p2align 2\n"                                                     \
          ".long (1b - 4b)\n"                                                \
+         ".if " #ptr_is_file "\n"                                           \
          ".long (2b - 4b)\n"                                                \
+         ".else\n"                                                          \
+         ".long (" #ptr " - 4b)\n"                                          \
+         ".endif\n"                                                         \
          ".long (3b - 4b)\n"                                                \
          ".hword " __stringify(line) ", 0\n"                                \
          ".popsection");                                                    \
 } while (0)
 
-#define WARN() BUG_FRAME(BUGFRAME_warn, __LINE__, __FILE__, 0, "")
+#define run_in_exception_handler(fn) BUG_FRAME(BUGFRAME_run_fn, 0, fn, 0, 0, 
"")
+
+#define WARN() BUG_FRAME(BUGFRAME_warn, __LINE__, __FILE__, 1, 0, "")
 
 #define BUG() do {                                              \
-    BUG_FRAME(BUGFRAME_bug,  __LINE__, __FILE__, 0, "");        \
+    BUG_FRAME(BUGFRAME_bug,  __LINE__, __FILE__, 1, 0, "");        \
     unreachable();                                              \
 } while (0)
 
 #define assert_failed(msg) do {                                 \
-    BUG_FRAME(BUGFRAME_assert, __LINE__, __FILE__, 1, msg);     \
+    BUG_FRAME(BUGFRAME_assert, __LINE__, __FILE__, 1, 1, msg);     \
     unreachable();                                              \
 } while (0)
 
-- 
2.26.2