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@xxxxxxxxxxxxx>
> Signed-off-by: Nikunj A Dadhania <nikunj@xxxxxxx>
> ---
> 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;
> + }
> +}
