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>
---
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 */

On 2025-10-29 09:22, Thomas Gleixner wrote:
> Guard the time slice extension functionality with a static key, which can
> be disabled on the kernel command line.

Reviewed-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>

-- 
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com

On 10/29/25 6:22 AM, Thomas Gleixner wrote:
> 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>
> ---
> 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/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;

The norm for __setup function returns is:

		return 0;	/* param not handled - will be added to ENV */
or
		return 1;	/* param is handled (anything non-zero) */


Anything non-zero means param is handled, so maybe -EINVAL is OK here,
since return 0 means that the string is added to init's environment.

If the parsing function recognizes the cmdline option string
(rseq_slice_ext) but the value is invalid, it should pr_error()
or something like that but still return 1; (IMHO).
No need to have "rseq_slice_ext=foo" added to init's ENV.

So return -EINVAL is like return 1 in this case.
IOW it works as needed.  :)

> +
> +	if (!on)
> +		static_branch_disable(&rseq_slice_extension_key);
> +	return 1;
> +}
> +__setup("rseq_slice_ext=", rseq_slice_cmdline);
> +#endif /* CONFIG_RSEQ_SLICE_EXTENSION */

-- 
~Randy

On Wed, Oct 29 2025 at 10:23, Randy Dunlap wrote:
> On 10/29/25 6:22 AM, Thomas Gleixner wrote:
>> +static int __init rseq_slice_cmdline(char *str)
>> +{
>> +	bool on;
>> +
>> +	if (kstrtobool(str, &on))
>> +		return -EINVAL;
>
> The norm for __setup function returns is:
>
> 		return 0;	/* param not handled - will be added to ENV */
> or
> 		return 1;	/* param is handled (anything non-zero) */
>
>
> Anything non-zero means param is handled, so maybe -EINVAL is OK here,
> since return 0 means that the string is added to init's environment.
>
> If the parsing function recognizes the cmdline option string
> (rseq_slice_ext) but the value is invalid, it should pr_error()
> or something like that but still return 1; (IMHO).
> No need to have "rseq_slice_ext=foo" added to init's ENV.
>
> So return -EINVAL is like return 1 in this case.
> IOW it works as needed.  :)

Bah. I hate this logic so much and I never will memorize it.