powerpc/pseries: Use jump labels for hcall tracepoints

hcall tracepoints add quite a few instructions to our hcall path:

plpar_hcall:
	mr      r2,r2
	mfcr    r0
	stw     r0,8(r1)
	b       164		<---- start
	ld      r12,0(r2)
	std     r12,32(r1)
	cmpdi   r12,0
	beq     164		<---- end
...

We have an unconditional branch that gets noped out during boot and
a load/compare/branch. We also store the tracepoint value to the
stack for the hcall_exit path to use.

By using jump labels we can simplify this to just a single nop that
gets replaced with a branch when the tracepoint is enabled:

plpar_hcall:
	mr      r2,r2
	mfcr    r0
	stw     r0,8(r1)
	nop			<----
...

If jump labels are not enabled, we fall back to the old method.

Signed-off-by: Anton Blanchard <anton@samba.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

1 files changed, 23 insertions, 7 deletions

diff --git a/arch/powerpc/platforms/pseries/lpar.c b/arch/powerpc/platforms/pseries/lpar.c
index b02af9ef3ff6..fbfcef514aa7 100644
--- a/arch/powerpc/platforms/pseries/lpar.c
+++ b/arch/powerpc/platforms/pseries/lpar.c
@@ -26,6 +26,7 @@
 #include <linux/dma-mapping.h>
 #include <linux/console.h>
 #include <linux/export.h>
+#include <linux/static_key.h>
 #include <asm/processor.h>
 #include <asm/mmu.h>
 #include <asm/page.h>
@@ -649,6 +650,19 @@ EXPORT_SYMBOL(arch_free_page);
 #endif
 
 #ifdef CONFIG_TRACEPOINTS
+#ifdef CONFIG_JUMP_LABEL
+struct static_key hcall_tracepoint_key = STATIC_KEY_INIT;
+
+void hcall_tracepoint_regfunc(void)
+{
+	static_key_slow_inc(&hcall_tracepoint_key);
+}
+
+void hcall_tracepoint_unregfunc(void)
+{
+	static_key_slow_dec(&hcall_tracepoint_key);
+}
+#else
 /*
  * We optimise our hcall path by placing hcall_tracepoint_refcount
  * directly in the TOC so we can check if the hcall tracepoints are
@@ -658,13 +672,6 @@ EXPORT_SYMBOL(arch_free_page);
 /* NB: reg/unreg are called while guarded with the tracepoints_mutex */
 extern long hcall_tracepoint_refcount;
 
-/* 
- * Since the tracing code might execute hcalls we need to guard against
- * recursion. One example of this are spinlocks calling H_YIELD on
- * shared processor partitions.
- */
-static DEFINE_PER_CPU(unsigned int, hcall_trace_depth);
-
 void hcall_tracepoint_regfunc(void)
 {
 	hcall_tracepoint_refcount++;
@@ -674,6 +681,15 @@ void hcall_tracepoint_unregfunc(void)
 {
 	hcall_tracepoint_refcount--;
 }
+#endif
+
+/*
+ * Since the tracing code might execute hcalls we need to guard against
+ * recursion. One example of this are spinlocks calling H_YIELD on
+ * shared processor partitions.
+ */
+static DEFINE_PER_CPU(unsigned int, hcall_trace_depth);
+
 
 void __trace_hcall_entry(unsigned long opcode, unsigned long *args)
 {