Invoke the DTrace trap handler before calling trap() on amd64. This matches

the upstream implementation and helps ensure that a trap induced by tracing
fbt::trap:entry is handled without recursively generating another trap.

This makes it possible to run most (but not all) of the DTrace tests under
common/safety/ without triggering a kernel panic.

Submitted by:	Anton Rang <anton.rang@isilon.com> (original version)
Phabric:	D95

2 files changed, 17 insertions, 12 deletions

diff --git a/sys/amd64/amd64/exception.S b/sys/amd64/amd64/exception.S
index d51078dd1808..bb5fd5669e74 100644
--- a/sys/amd64/amd64/exception.S
+++ b/sys/amd64/amd64/exception.S
@@ -228,7 +228,24 @@ alltraps_pushregs_no_rdi:
 	.type	calltrap,@function
 calltrap:
 	movq	%rsp,%rdi
+#ifdef KDTRACE_HOOKS
+	/*
+	 * Give DTrace a chance to vet this trap and skip the call to trap() if
+	 * it turns out that it was caused by a DTrace probe.
+	 */
+	movq	dtrace_trap_func,%rax
+	testq	%rax,%rax
+	je	skiphook
+	call	*%rax
+	testq	%rax,%rax
+	jne	skiptrap
+	movq	%rsp,%rdi
+skiphook:
+#endif
 	call	trap
+#ifdef KDTRACE_HOOKS
+skiptrap:
+#endif
 	MEXITCOUNT
 	jmp	doreti			/* Handle any pending ASTs */
 
diff --git a/sys/amd64/amd64/trap.c b/sys/amd64/amd64/trap.c
index c56b99e434b1..d9203bc5e7c8 100644
--- a/sys/amd64/amd64/trap.c
+++ b/sys/amd64/amd64/trap.c
@@ -218,18 +218,6 @@ trap(struct trapframe *frame)
 		goto out;
 	}
 
-#ifdef KDTRACE_HOOKS
-	/*
-	 * A trap can occur while DTrace executes a probe. Before
-	 * executing the probe, DTrace blocks re-scheduling and sets
-	 * a flag in its per-cpu flags to indicate that it doesn't
-	 * want to fault. On returning from the probe, the no-fault
-	 * flag is cleared and finally re-scheduling is enabled.
-	 */
-	if (dtrace_trap_func != NULL && (*dtrace_trap_func)(frame, type))
-		goto out;
-#endif
-
 	if ((frame->tf_rflags & PSL_I) == 0) {
 		/*
 		 * Buggy application or kernel code has disabled