From: Thomas Gleixner <tglx@linutronix.de>
Date: Mon, 15 Dec 2025 16:52:06 +0000 (+0100)
Subject: rseq: Provide static branch for time slice extensions
X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=f8380f976804533df4c6c3d3a0b2cd03c2d262bc;p=thirdparty%2Fkernel%2Flinux.git

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@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://patch.msgid.link/20251215155708.733429292@linutronix.de
---

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index a8d0afde7f85a..f2348bca36a16 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -6600,6 +6600,11 @@ Kernel parameters
 
 	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.
 
diff --git a/include/linux/rseq_entry.h b/include/linux/rseq_entry.h
index a36b472627dec..d0ec4714ffd61 100644
--- a/include/linux/rseq_entry.h
+++ b/include/linux/rseq_entry.h
@@ -75,6 +75,17 @@ DECLARE_STATIC_KEY_MAYBE(CONFIG_RSEQ_DEBUG_DEFAULT_ENABLE, rseq_debug_enabled);
 #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);
 
diff --git a/kernel/rseq.c b/kernel/rseq.c
index 07c324d5a201a..bf75268580ef2 100644
--- a/kernel/rseq.c
+++ b/kernel/rseq.c
@@ -483,3 +483,20 @@ SYSCALL_DEFINE4(rseq, struct rseq __user *, rseq, u32, rseq_len, int, flags, u32
 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 0;
+
+	if (!on)
+		static_branch_disable(&rseq_slice_extension_key);
+	return 1;
+}
+__setup("rseq_slice_ext=", rseq_slice_cmdline);
+#endif /* CONFIG_RSEQ_SLICE_EXTENSION */