[PATCH v4 09/17] perf/core: Use static_call to optimize perf_guest_info_callbacks

[Sean Christopherson]

Use static_call to optimize perf's guest callbacks on arm64 and x86,
which are now the only architectures that define the callbacks.  Use
DEFINE_STATIC_CALL_RET0 as the default/NULL for all guest callbacks, as
the callback semantics are that a return value '0' means "not in guest".

static_call obviously avoids the overhead of CONFIG_RETPOLINE=y, but is
also advantageous versus other solutions, e.g. per-cpu callbacks, in that
a per-cpu memory load is not needed to detect the !guest case.

Based on code from Peter and Like.

Suggested-by: Peter Zijlstra (Intel) <peterz at infradead.org>
Cc: Like Xu <like.xu.linux at gmail.com>
Reviewed-by: Paolo Bonzini <pbonzini at redhat.com>
Signed-off-by: Sean Christopherson <seanjc at google.com>
---
 include/linux/perf_event.h | 34 ++++++++--------------------------
 kernel/events/core.c       | 15 +++++++++++++++
 2 files changed, 23 insertions(+), 26 deletions(-)

diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
index ea47ef616ee0..0ac7d867ca0c 100644
--- a/include/linux/perf_event.h
+++ b/include/linux/perf_event.h
@@ -1244,40 +1244,22 @@ extern void perf_event_bpf_event(struct bpf_prog *prog,
 
 #ifdef CONFIG_GUEST_PERF_EVENTS
 extern struct perf_guest_info_callbacks __rcu *perf_guest_cbs;
-static inline struct perf_guest_info_callbacks *perf_get_guest_cbs(void)
-{
-	/*
-	 * Callbacks are RCU-protected and must be READ_ONCE to avoid reloading
-	 * the callbacks between a !NULL check and dereferences, to ensure
-	 * pending stores/changes to the callback pointers are visible before a
-	 * non-NULL perf_guest_cbs is visible to readers, and to prevent a
-	 * module from unloading callbacks while readers are active.
-	 */
-	return rcu_dereference(perf_guest_cbs);
-}
+
+DECLARE_STATIC_CALL(__perf_guest_state, *perf_guest_cbs->state);
+DECLARE_STATIC_CALL(__perf_guest_get_ip, *perf_guest_cbs->get_ip);
+DECLARE_STATIC_CALL(__perf_guest_handle_intel_pt_intr, *perf_guest_cbs->handle_intel_pt_intr);
+
 static inline unsigned int perf_guest_state(void)
 {
-	struct perf_guest_info_callbacks *guest_cbs = perf_get_guest_cbs();
-
-	return guest_cbs ? guest_cbs->state() : 0;
+	return static_call(__perf_guest_state)();
 }
 static inline unsigned long perf_guest_get_ip(void)
 {
-	struct perf_guest_info_callbacks *guest_cbs = perf_get_guest_cbs();
-
-	/*
-	 * Arbitrarily return '0' in the unlikely scenario that the callbacks
-	 * are unregistered between checking guest state and getting the IP.
-	 */
-	return guest_cbs ? guest_cbs->get_ip() : 0;
+	return static_call(__perf_guest_get_ip)();
 }
 static inline unsigned int perf_guest_handle_intel_pt_intr(void)
 {
-	struct perf_guest_info_callbacks *guest_cbs = perf_get_guest_cbs();
-
-	if (guest_cbs && guest_cbs->handle_intel_pt_intr)
-		return guest_cbs->handle_intel_pt_intr();
-	return 0;
+	return static_call(__perf_guest_handle_intel_pt_intr)();
 }
 extern void perf_register_guest_info_callbacks(struct perf_guest_info_callbacks *cbs);
 extern void perf_unregister_guest_info_callbacks(struct perf_guest_info_callbacks *cbs);
diff --git a/kernel/events/core.c b/kernel/events/core.c
index 1c8d341ecc77..b4fd928e4ff8 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -6524,12 +6524,23 @@ static void perf_pending_event(struct irq_work *entry)
 #ifdef CONFIG_GUEST_PERF_EVENTS
 struct perf_guest_info_callbacks __rcu *perf_guest_cbs;
 
+DEFINE_STATIC_CALL_RET0(__perf_guest_state, *perf_guest_cbs->state);
+DEFINE_STATIC_CALL_RET0(__perf_guest_get_ip, *perf_guest_cbs->get_ip);
+DEFINE_STATIC_CALL_RET0(__perf_guest_handle_intel_pt_intr, *perf_guest_cbs->handle_intel_pt_intr);
+
 void perf_register_guest_info_callbacks(struct perf_guest_info_callbacks *cbs)
 {
 	if (WARN_ON_ONCE(rcu_access_pointer(perf_guest_cbs)))
 		return;
 
 	rcu_assign_pointer(perf_guest_cbs, cbs);
+	static_call_update(__perf_guest_state, cbs->state);
+	static_call_update(__perf_guest_get_ip, cbs->get_ip);
+
+	/* Implementing ->handle_intel_pt_intr is optional. */
+	if (cbs->handle_intel_pt_intr)
+		static_call_update(__perf_guest_handle_intel_pt_intr,
+				   cbs->handle_intel_pt_intr);
 }
 EXPORT_SYMBOL_GPL(perf_register_guest_info_callbacks);
 
@@ -6539,6 +6550,10 @@ void perf_unregister_guest_info_callbacks(struct perf_guest_info_callbacks *cbs)
 		return;
 
 	rcu_assign_pointer(perf_guest_cbs, NULL);
+	static_call_update(__perf_guest_state, (void *)&__static_call_return0);
+	static_call_update(__perf_guest_get_ip, (void *)&__static_call_return0);
+	static_call_update(__perf_guest_handle_intel_pt_intr,
+			   (void *)&__static_call_return0);
 	synchronize_rcu();
 }
 EXPORT_SYMBOL_GPL(perf_unregister_guest_info_callbacks);
-- 
2.34.0.rc0.344.g81b53c2807-goog