Re: [PATCH 5/5] x86/tsc: Make __use_tsc __ro_after_init

[Peter Zijlstra <peterz@infradead.org>]

On Mon, Nov 20, 2023 at 11:55:28AM +0100, Valentin Schneider wrote:
> __use_tsc is only ever enabled in __init tsc_enable_sched_clock(), so mark
> it as __ro_after_init.
> 
> Signed-off-by: Valentin Schneider <vschneid@redhat.com>
> ---
>  arch/x86/kernel/tsc.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c
> index 15f97c0abc9d0..f19b42ea40573 100644
> --- a/arch/x86/kernel/tsc.c
> +++ b/arch/x86/kernel/tsc.c
> @@ -44,7 +44,7 @@ EXPORT_SYMBOL(tsc_khz);
>  static int __read_mostly tsc_unstable;
>  static unsigned int __initdata tsc_early_khz;
>  
> -static DEFINE_STATIC_KEY_FALSE(__use_tsc);
> +static DEFINE_STATIC_KEY_FALSE_RO(__use_tsc);

So sure, we can absolutely do that, but do we want to take this one
further perhaps? "notsc" on x86_64 makes no sense what so ever. Lets
drag things into this millennium.

---

diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c
index 15f97c0abc9d..4cfcf5946162 100644
--- a/arch/x86/kernel/tsc.c
+++ b/arch/x86/kernel/tsc.c
@@ -44,7 +44,9 @@ EXPORT_SYMBOL(tsc_khz);
 static int __read_mostly tsc_unstable;
 static unsigned int __initdata tsc_early_khz;
 
+#ifndef CONFIG_X86_64
 static DEFINE_STATIC_KEY_FALSE(__use_tsc);
+#endif
 
 int tsc_clocksource_reliable;
 
@@ -230,24 +232,26 @@ static void __init cyc2ns_init_secondary_cpus(void)
  */
 noinstr u64 native_sched_clock(void)
 {
-	if (static_branch_likely(&__use_tsc)) {
-		u64 tsc_now = rdtsc();
+#ifndef CONFIG_X86_64
+	if (!static_branch_unlikely(&__use_tsc)) {
+		/*
+		 * Fall back to jiffies if there's no TSC available:
+		 * ( But note that we still use it if the TSC is marked
+		 *   unstable. We do this because unlike Time Of Day,
+		 *   the scheduler clock tolerates small errors and it's
+		 *   very important for it to be as fast as the platform
+		 *   can achieve it. )
+		 */
 
-		/* return the value in ns */
-		return __cycles_2_ns(tsc_now);
+		/* No locking but a rare wrong value is not a big deal: */
+		return (jiffies_64 - INITIAL_JIFFIES) * (1000000000 / HZ);
 	}
+#endif
 
-	/*
-	 * Fall back to jiffies if there's no TSC available:
-	 * ( But note that we still use it if the TSC is marked
-	 *   unstable. We do this because unlike Time Of Day,
-	 *   the scheduler clock tolerates small errors and it's
-	 *   very important for it to be as fast as the platform
-	 *   can achieve it. )
-	 */
+	u64 tsc_now = rdtsc();
 
-	/* No locking but a rare wrong value is not a big deal: */
-	return (jiffies_64 - INITIAL_JIFFIES) * (1000000000 / HZ);
+	/* return the value in ns */
+	return __cycles_2_ns(tsc_now);
 }
 
 /*
@@ -291,7 +295,8 @@ int check_tsc_unstable(void)
 }
 EXPORT_SYMBOL_GPL(check_tsc_unstable);
 
-#ifdef CONFIG_X86_TSC
+#ifndef CONFIG_X86_64
+#if defined(CONFIG_X86_TSC
 int __init notsc_setup(char *str)
 {
 	mark_tsc_unstable("boot parameter notsc");
@@ -310,6 +315,7 @@ int __init notsc_setup(char *str)
 #endif
 
 __setup("notsc", notsc_setup);
+#endif
 
 static int no_sched_irq_time;
 static int no_tsc_watchdog;
@@ -1556,7 +1562,9 @@ static void __init tsc_enable_sched_clock(void)
 	/* Sanitize TSC ADJUST before cyc2ns gets initialized */
 	tsc_store_and_check_tsc_adjust(true);
 	cyc2ns_init_boot_cpu();
+#ifndef CONFIG_X86_64
 	static_branch_enable(&__use_tsc);
+#endif
 }
 
 void __init tsc_early_init(void)