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

[Randy Dunlap]

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@xxxxxxxxxxxxx>
> Cc: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Cc: "Paul E. McKenney" <paulmck@xxxxxxxxxx>
> Cc: Boqun Feng <boqun.feng@xxxxxxxxx>
> ---
> 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