linux-kernel - Re: [PATCH v14 08/25] x86/kvmclock: Avoid TSC recalibration

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Message-ID: <ddda16df-cca6-8b9d-6f66-6ff5ecc21a03@redhat.com>
Date:   Wed, 18 Jul 2018 13:14:02 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Pavel Tatashin <pasha.tatashin@...cle.com>,
        steven.sistare@...cle.com, daniel.m.jordan@...cle.com,
        linux@...linux.org.uk, schwidefsky@...ibm.com,
        heiko.carstens@...ibm.com, john.stultz@...aro.org,
        sboyd@...eaurora.org, x86@...nel.org, linux-kernel@...r.kernel.org,
        mingo@...hat.com, tglx@...utronix.de, hpa@...or.com,
        douly.fnst@...fujitsu.com, peterz@...radead.org, prarit@...hat.com,
        feng.tang@...el.com, pmladek@...e.com, gnomes@...rguk.ukuu.org.uk,
        linux-s390@...r.kernel.org, boris.ostrovsky@...cle.com,
        jgross@...e.com
Subject: Re: [PATCH v14 08/25] x86/kvmclock: Avoid TSC recalibration

On 18/07/2018 04:21, Pavel Tatashin wrote:
> From: Peter Zijlstra <peterz@...radead.org>
> 
> If the host gives us a TSC rate, assume it is good and don't try and
> recalibrate things against virtual timer hardware.
> 
> Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> Signed-off-by: Pavel Tatashin <pasha.tatashin@...cle.com>
> ---
>  arch/x86/kernel/kvmclock.c | 11 ++++++++++-
>  1 file changed, 10 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kernel/kvmclock.c b/arch/x86/kernel/kvmclock.c
> index ed170171fe49..da0ede8ac8f6 100644
> --- a/arch/x86/kernel/kvmclock.c
> +++ b/arch/x86/kernel/kvmclock.c
> @@ -141,7 +141,16 @@ static inline void kvm_sched_clock_init(bool stable)
>   */
>  static unsigned long kvm_get_tsc_khz(void)
>  {
> -	return pvclock_tsc_khz(this_cpu_pvti());
> +	unsigned long tsc_khz = pvclock_tsc_khz(this_cpu_pvti());
> +
> +	/*
> +	 * TSC frequency is reported by the host; calibration against (virtual)
> +	 * HPET/PM-timer in a guest is dodgy and pointless since the host
> +	 * already did it for us where required.
> +	 */
> +	setup_force_cpu_cap(X86_FEATURE_TSC_KNOWN_FREQ);
> +
> +	return tsc_khz;
>  }
>  
>  static void kvm_get_preset_lpj(void)
> 

This patch (really a similar one) has just been sent to Linus.

Paolo