[patch 03/12] rseq: Provide static branch for time slice extensions

[Thomas Gleixner <tglx@linutronix.de>]

Guard the time slice extension functionality with a static key, which can
be disabled on the kernel command line.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: "Paul E. McKenney" <paulmck@kernel.org>
Cc: Boqun Feng <boqun.feng@gmail.com>
---
 include/linux/rseq_entry.h |   11 +++++++++++
 kernel/rseq.c              |   17 +++++++++++++++++
 2 files changed, 28 insertions(+)

--- a/include/linux/rseq_entry.h
+++ b/include/linux/rseq_entry.h
@@ -77,6 +77,17 @@ DECLARE_STATIC_KEY_MAYBE(CONFIG_RSEQ_DEB
 #define rseq_inline __always_inline
 #endif
 
+#ifdef CONFIG_RSEQ_SLICE_EXTENSION
+DECLARE_STATIC_KEY_TRUE(rseq_slice_extension_key);
+
+static __always_inline bool rseq_slice_extension_enabled(void)
+{
+	return static_branch_likely(&rseq_slice_extension_key);
+}
+#else /* CONFIG_RSEQ_SLICE_EXTENSION */
+static inline bool rseq_slice_extension_enabled(void) { return false; }
+#endif /* !CONFIG_RSEQ_SLICE_EXTENSION */
+
 bool rseq_debug_update_user_cs(struct task_struct *t, struct pt_regs *regs, unsigned long csaddr);
 bool rseq_debug_validate_ids(struct task_struct *t);
 
--- a/kernel/rseq.c
+++ b/kernel/rseq.c
@@ -474,3 +474,20 @@ SYSCALL_DEFINE4(rseq, struct rseq __user
 
 	return 0;
 }
+
+#ifdef CONFIG_RSEQ_SLICE_EXTENSION
+DEFINE_STATIC_KEY_TRUE(rseq_slice_extension_key);
+
+static int __init rseq_slice_cmdline(char *str)
+{
+	bool on;
+
+	if (kstrtobool(str, &on))
+		return -EINVAL;
+
+	if (!on)
+		static_branch_disable(&rseq_slice_extension_key);
+	return 0;
+}
+__setup("rseq_slice_ext=", rseq_slice_cmdline);
+#endif /* CONFIG_RSEQ_SLICE_EXTENSION */