linux-kernel - Re: [PATCH v12 15/19] tsc: Upgrade TSC clocksource rating

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Message-ID: <ZwasXk_jZAtO6W1G@google.com>
Date: Wed, 9 Oct 2024 09:16:30 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Nikunj A Dadhania <nikunj@....com>
Cc: linux-kernel@...r.kernel.org, thomas.lendacky@....com, bp@...en8.de, 
	x86@...nel.org, kvm@...r.kernel.org, mingo@...hat.com, tglx@...utronix.de, 
	dave.hansen@...ux.intel.com, pgonda@...gle.com, pbonzini@...hat.com
Subject: Re: [PATCH v12 15/19] tsc: Upgrade TSC clocksource rating

On Wed, Oct 09, 2024, Nikunj A Dadhania wrote:
> In virtualized environments running on modern CPUs, the underlying
> platforms guarantees to have a stable, always running TSC, i.e. that the
> TSC is a superior timesource as compared to other clock sources (such as
> kvmclock, HPET, ACPI timer, APIC, etc.).
> 
> Upgrade the rating of the early and regular clock source to prefer TSC over
> other clock sources when TSC is invariant, non-stop and stable.
> 
> Suggested-by: Thomas Gleixner <tglx@...utronix.de>
> Signed-off-by: Nikunj A Dadhania <nikunj@....com>
> ---
>  arch/x86/kernel/tsc.c | 17 +++++++++++++++++
>  1 file changed, 17 insertions(+)
> 
> diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c
> index c83f1091bb4f..8150f2104474 100644
> --- a/arch/x86/kernel/tsc.c
> +++ b/arch/x86/kernel/tsc.c
> @@ -1264,6 +1264,21 @@ static void __init check_system_tsc_reliable(void)
>  		tsc_disable_clocksource_watchdog();
>  }
>  
> +static void __init upgrade_clock_rating(struct clocksource *tsc_early,
> +					struct clocksource *tsc)
> +{
> +	/*
> +	 * Upgrade the clock rating for TSC early and regular clocksource when
> +	 * the underlying platform provides non-stop, invaraint and stable TSC.
> +	 */
> +	if (boot_cpu_has(X86_FEATURE_CONSTANT_TSC) &&
> +	    boot_cpu_has(X86_FEATURE_NONSTOP_TSC) &&
> +	    !tsc_unstable) {

Somewhat of a side topic, should KVM (as a hypervisor) be enumerating something
to guests to inform them that the TSC is reliable, i.e. that X86_FEATURE_TSC_RELIABLE
can be forced?  Or, should KVM (as the guest) infer X86_FEATURE_TSC_RELIABLE if
INVARIANT_TSC is advertised by KVM (the hyperivosor)?

Also, why on earth is 0x8000_0007.EDX manually scattered via x86_power?

> +		tsc_early->rating = 499;
> +		tsc->rating = 500;
> +	}
> +}