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

  __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(-)
  

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)
  

On 20/11/23 13:05, Peter Zijlstra wrote:
> 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.
>

Just to make sure I follow: currently, for the static key to be enabled, we
(mostly) need:
o X86_FEATURE_TSC is in CPUID
o determine_cpu_tsc_frequencies()->pit_hpet_ptimer_calibrate_cpu() passes

IIUC all X86_64 systems have a TSC, so the CPUID feature should be a given.

AFAICT pit_hpt_ptimer_calibrate_cpu() relies on having either HPET or the
ACPI PM timer, the latter should be widely available, though X86_PM_TIMER
can be disabled via EXPERT - is that a fringe case we don't care about, or
did I miss something? I don't really know this stuff, and I'm trying to
write a changelog...
  

On Mon, Dec 04, 2023 at 05:51:49PM +0100, Valentin Schneider wrote:
> On 20/11/23 13:05, Peter Zijlstra wrote:
> > 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.
> >
> 
> Just to make sure I follow: currently, for the static key to be enabled, we
> (mostly) need:
> o X86_FEATURE_TSC is in CPUID
> o determine_cpu_tsc_frequencies()->pit_hpet_ptimer_calibrate_cpu() passes
> 
> IIUC all X86_64 systems have a TSC, so the CPUID feature should be a given.
> 
> AFAICT pit_hpt_ptimer_calibrate_cpu() relies on having either HPET or the
> ACPI PM timer, the latter should be widely available, though X86_PM_TIMER
> can be disabled via EXPERT - is that a fringe case we don't care about, or
> did I miss something? I don't really know this stuff, and I'm trying to
> write a changelog...

Ah, I was mostly just going by the fact that all of x86_64 have TSC and
disabling it makes no sense.

TSC calibration is always 'fun', but I don't know of a system where its
failure causes us to not use TSC, Thomas?
  

On 04/12/23 19:20, Peter Zijlstra wrote:
> On Mon, Dec 04, 2023 at 05:51:49PM +0100, Valentin Schneider wrote:
>> On 20/11/23 13:05, Peter Zijlstra wrote:
>> > 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.
>> >
>>
>> Just to make sure I follow: currently, for the static key to be enabled, we
>> (mostly) need:
>> o X86_FEATURE_TSC is in CPUID
>> o determine_cpu_tsc_frequencies()->pit_hpet_ptimer_calibrate_cpu() passes
>>
>> IIUC all X86_64 systems have a TSC, so the CPUID feature should be a given.
>>
>> AFAICT pit_hpt_ptimer_calibrate_cpu() relies on having either HPET or the
>> ACPI PM timer, the latter should be widely available, though X86_PM_TIMER
>> can be disabled via EXPERT - is that a fringe case we don't care about, or
>> did I miss something? I don't really know this stuff, and I'm trying to
>> write a changelog...
>
> Ah, I was mostly just going by the fact that all of x86_64 have TSC and
> disabling it makes no sense.
>
> TSC calibration is always 'fun', but I don't know of a system where its
> failure causes us to not use TSC, Thomas?

Having another look at this, it looks like the actual requirements for the
TSC being used are either of:
o CPUID accepting 0x16 as eax input (cf. cpu_khz_from_cpuid())
o MSR_FSB_FREQ being available (cf. cpu_khz_from_msr())
o pit_hpet_ptimer_calibrate_cpu() doesn't mess up

I couldn't find any guarantees for x86_64 on having the processor frequency
information CPUID leaf, nor for the FSB_FREQ MSR (both tsc_msr_cpu_ids and
the SDM seem to point at only a handful of models).

Also for x86_64 there is this "apicpmtimer" cmdline arg which currently
disables the TSC. The commit that introduced it [1] suggests there are x86_64
systems out there with calibration issues, so now I'm not sure whether we
can kill the static key for x86_64 :(

[1]: 0c3749c41f5e ("[PATCH] x86_64: Calibrate APIC timer using PM timer")
followed by: 7fd67843b96f ("[PATCH] x86_64: Disable tsc when apicpmtimer is active")