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

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Message-ID: <xhsmhmstnkc0g.mognet@vschneid-thinkpadt14sgen2i.remote.csb>
Date: Tue, 02 Jan 2024 16:09:35 +0100
From: Valentin Schneider <vschneid@...hat.com>
To: Peter Zijlstra <peterz@...radead.org>
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 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@...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?

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")