linux-kernel - [patch V3 03/12] rseq: Provide static branch for time slice extensions

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Message-ID: <20251029130403.606732100@linutronix.de>
Date: Wed, 29 Oct 2025 14:22:16 +0100 (CET)
From: Thomas Gleixner <tglx@...utronix.de>
To: LKML <linux-kernel@...r.kernel.org>
Cc: Peter Zijlstra <peterz@...radead.org>,
 Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
 "Paul E. McKenney" <paulmck@...nel.org>,
 Boqun Feng <boqun.feng@...il.com>,
 Jonathan Corbet <corbet@....net>,
 Prakash Sangappa <prakash.sangappa@...cle.com>,
 Madadi Vineeth Reddy <vineethr@...ux.ibm.com>,
 K Prateek Nayak <kprateek.nayak@....com>,
 Steven Rostedt <rostedt@...dmis.org>,
 Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
 Arnd Bergmann <arnd@...db.de>,
 linux-arch@...r.kernel.org
Subject: [patch V3 03/12] rseq: Provide static branch for time slice
 extensions

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@...utronix.de>
Cc: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: "Paul E. McKenney" <paulmck@...nel.org>
Cc: Boqun Feng <boqun.feng@...il.com>
---
V3: Document command line parameter - Sebastian
V2: Return 1 from __setup() - Prateek
---
 Documentation/admin-guide/kernel-parameters.txt |    5 +++++
 include/linux/rseq_entry.h                      |   11 +++++++++++
 kernel/rseq.c                                   |   17 +++++++++++++++++
 3 files changed, 33 insertions(+)

--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -6482,6 +6482,11 @@
 
 	rootflags=	[KNL] Set root filesystem mount option string
 
+	rseq_slice_ext= [KNL] RSEQ based time slice extension
+			Format: boolean
+			Control enablement of RSEQ based time slice extension.
+			Default is 'on'.
+
 	initramfs_options= [KNL]
                         Specify mount options for for the initramfs mount.
 
--- a/include/linux/rseq_entry.h
+++ b/include/linux/rseq_entry.h
@@ -75,6 +75,17 @@ 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
@@ -484,3 +484,20 @@ SYSCALL_DEFINE4(rseq, struct rseq __user
 efault:
 	return -EFAULT;
 }
+
+#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 1;
+}
+__setup("rseq_slice_ext=", rseq_slice_cmdline);
+#endif /* CONFIG_RSEQ_SLICE_EXTENSION */