linux-kernel - Re: [PATCH 5/5] x86/tsc: Make __use_tsc __ro_after

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Date:   Mon, 4 Dec 2023 19:20:43 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Valentin Schneider <vschneid@...hat.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        linux-arch@...r.kernel.org, x86@...nel.org,
        Thomas Gleixner <tglx@...utronix.de>,
        Borislav Petkov <bp@...en8.de>,
        Josh Poimboeuf <jpoimboe@...nel.org>,
        Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>,
        Ingo Molnar <mingo@...hat.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Arnd Bergmann <arnd@...db.de>, Jason Baron <jbaron@...mai.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ard Biesheuvel <ardb@...nel.org>,
        Frederic Weisbecker <frederic@...nel.org>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Feng Tang <feng.tang@...el.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Mike Rapoport (IBM)" <rppt@...nel.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        David Hildenbrand <david@...hat.com>,
        "ndesaulniers@...gle.com" <ndesaulniers@...gle.com>,
        Michael Kelley <mikelley@...rosoft.com>,
        "Masami Hiramatsu (Google)" <mhiramat@...nel.org>
Subject: Re: [PATCH 5/5] x86/tsc: Make __use_tsc __ro_after_init

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@...hat.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?