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@xxxxxxxxxx> >> --- >> 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...
