kernel: Let user_debugger take care of tracking syscall runtime.

If we just use the kernel entry time, then the pre-syscall tracing
routine (with a debugger message send) will be counted in the syscall's
runtime.

Makes the output of timing in strace and strace -c much more accurate,
however it won't include the "syscall overhead" (time spent in the
syscall entry routines, etc.) But we already can't account for time
spent in the userland-to-kernel transition, so that should probably
be measured some other way if knowing it is desired.

In fact, on architectures which used the generic syscall dispatcher
(e.g. RISC-V), this is the behavior that already existed. So this just
makes x86 consistent with them.

Change-Id: I8cef6111e478ab49b0584e15575172eea77a8760
Reviewed-on: https://review.haiku-os.org/c/haiku/+/8240
Tested-by: Commit checker robot <[email protected]>
Reviewed-by: waddlesplash <[email protected]>

headers/private/kernel/user_debugger.h

@@ -122,8 +122,10 @@ struct thread_debug_info {
 
 	// profiling related part; if samples != NULL, the thread is profiled
 	struct {
-		bigtime_t		interval;
-			// sampling interval
+		union {
+			bigtime_t		interval;
+			bigtime_t		syscall_start_time;
+		};
 		area_id			sample_area;
 			// cloned sample buffer area
 		addr_t*			samples;
@@ -253,8 +255,7 @@ void init_user_debug();
 // debug event callbacks
 
 void user_debug_pre_syscall(uint32 syscall, void *args);
-void user_debug_post_syscall(uint32 syscall, void *args, uint64 returnValue,
-		bigtime_t startTime);
+void user_debug_post_syscall(uint32 syscall, void *args, uint64 returnValue);
 bool user_debug_exception_occurred(debug_exception_type exception, int signal);
 bool user_debug_handle_signal(int signal, struct sigaction *handler,
 		siginfo_t *info, bool deadly);