Make platform-specific Feather-Trace depend on !CONFIG_DEBUG_RODATA

Feather-Trace rewrites instructions in the kernel's .text segment.
This segment may be write-protected if CONFIG_DEBUG_RODATA is selected.
In this case, fall back to the default flag-based Feather-Trace
implementation. In the future, we could either adopt the ftrace method
of rewriting .text addresses using non-.text mappings or we could
consider replacing Feather-Trace with ftrace altogether.

For now, this patch avoids unexpected runtime errors.

4 files changed, 26 insertions, 2 deletions

diff --git a/arch/x86/kernel/ft_event.c b/arch/x86/kernel/ft_event.c
index da9b3ccb7259..6552e2741ecf 100644
--- a/arch/x86/kernel/ft_event.c
+++ b/arch/x86/kernel/ft_event.c
@@ -6,6 +6,7 @@
  * exclusive access to the event table
  */
 
+#ifndef CONFIG_DEBUG_RODATA
 
 #define BYTE_JUMP      0xeb
 #define BYTE_JUMP_LEN  0x02
@@ -106,3 +107,5 @@ int ft_is_event_enabled(unsigned long id)
         }
         return 0;
 }
+
+#endif
diff --git a/include/litmus/feather_trace.h b/include/litmus/feather_trace.h
index f3bbfa0935ef..028dfb206fb0 100644
--- a/include/litmus/feather_trace.h
+++ b/include/litmus/feather_trace.h
@@ -19,7 +19,12 @@ static inline int fetch_and_dec(int *val)
         return atomic_sub_return(1, (atomic_t*) val) + 1;
 }
 
-#ifdef CONFIG_ARCH_HAS_FEATHER_TRACE
+/* Don't use rewriting implementation if kernel text pages are read-only.
+ * Ftrace gets around this by using the identity mapping, but that's more
+ * effort that is warrented right now for Feather-Trace.
+ * Eventually, it may make sense to replace Feather-Trace with ftrace.
+ */
+#if defined(CONFIG_ARCH_HAS_FEATHER_TRACE) && !defined(CONFIG_DEBUG_RODATA)
 
 #include <asm/feather_trace.h>
 
@@ -27,6 +32,13 @@ static inline int fetch_and_dec(int *val)
 
 /* provide default implementation */
 
+#include <asm/timex.h> /* for get_cycles() */
+
+static inline unsigned long long ft_timestamp(void)
+{
+        return get_cycles();
+}
+
 #define feather_callback
 
 #define MAX_EVENTS 1024
diff --git a/litmus/Kconfig b/litmus/Kconfig
index 1c0a96d54992..f3aa7478faf7 100644
--- a/litmus/Kconfig
+++ b/litmus/Kconfig
@@ -80,6 +80,15 @@ config FEATHER_TRACE
           Feather-Trace basic tracing infrastructure. Includes device file
           driver and instrumentation point support.
 
+          There are actually two implementations of Feather-Trace.
+          1) A slower, but portable, default implementation.
+          2) Architecture-specific implementations that rewrite kernel .text at runtime.
+
+          If enabled, Feather-Trace will be based on 2) if available (currently only for x86).
+          However, if DEBUG_RODATA=y, then Feather-Trace will choose option 1) in any case
+          to avoid problems with write-protected .text pages.
+
+          Bottom line: to avoid increased overheads, choose DEBUG_RODATA=n.
 
 config SCHED_TASK_TRACE
         bool "Trace real-time tasks"
diff --git a/litmus/ft_event.c b/litmus/ft_event.c
index 58124fe0ad90..399a07becca5 100644
--- a/litmus/ft_event.c
+++ b/litmus/ft_event.c
@@ -2,7 +2,7 @@
 
 #include <litmus/feather_trace.h>
 
-#ifndef CONFIG_ARCH_HAS_FEATHER_TRACE
+#if !defined(CONFIG_ARCH_HAS_FEATHER_TRACE) || defined(CONFIG_DEBUG_RODATA)
 /* provide dummy implementation */
 
 int ft_events[MAX_EVENTS];