x86: print relevant (tail) part of filename for warnings and crashes

In particular when the origin construct is in a header file (and
hence the file name is an absolute path instead of just the file name
portion) the information can otherwise become rather useless when the
build tree isn't sitting relatively close to the file system root.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

--- a/xen/arch/x86/traps.c
+++ b/xen/arch/x86/traps.c
@@ -945,7 +945,7 @@ void do_invalid_op(struct cpu_user_regs 
 {
     const struct bug_frame *bug;
     u8 bug_insn[2];
-    const char *filename, *predicate, *eip = (char *)regs->eip;
+    const char *prefix = "", *filename, *predicate, *eip = (char *)regs->eip;
     unsigned long fixup;
     int id, lineno;
     static const struct bug_frame *const stop_frames[] = {
@@ -996,21 +996,27 @@ void do_invalid_op(struct cpu_user_regs 
     filename = bug_ptr(bug);
     if ( !is_kernel(filename) )
         goto die;
+    fixup = strlen(filename);
+    if ( fixup > 50 )
+    {
+        filename += fixup - 47;
+        prefix = "...";
+    }
     lineno = bug_line(bug);
 
     switch ( id )
     {
     case BUGFRAME_warn:
-        printk("Xen WARN at %.50s:%d\n", filename, lineno);
+        printk("Xen WARN at %s%s:%d\n", prefix, filename, lineno);
         show_execution_state(regs);
         regs->eip = (unsigned long)eip;
         return;
 
     case BUGFRAME_bug:
-        printk("Xen BUG at %.50s:%d\n", filename, lineno);
+        printk("Xen BUG at %s%s:%d\n", prefix, filename, lineno);
         DEBUGGER_trap_fatal(TRAP_invalid_op, regs);
         show_execution_state(regs);
-        panic("Xen BUG at %.50s:%d\n", filename, lineno);
+        panic("Xen BUG at %s%s:%d\n", prefix, filename, lineno);
 
     case BUGFRAME_assert:
         /* ASSERT: decode the predicate string pointer. */
@@ -1018,12 +1024,12 @@ void do_invalid_op(struct cpu_user_regs 
         if ( !is_kernel(predicate) )
             predicate = "<unknown>";
 
-        printk("Assertion '%s' failed at %.50s:%d\n",
-               predicate, filename, lineno);
+        printk("Assertion '%s' failed at %s%s:%d\n",
+               predicate, prefix, filename, lineno);
         DEBUGGER_trap_fatal(TRAP_invalid_op, regs);
         show_execution_state(regs);
-        panic("Assertion '%s' failed at %.50s:%d\n",
-              predicate, filename, lineno);
+        panic("Assertion '%s' failed at %s%s:%d\n",
+              predicate, prefix, filename, lineno);
     }
 
  die: