linux-kernel - RE: [PATCH] clocksource: hyper-v: make sure Invariant-TSC is used if it is available

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Date:   Tue, 11 Apr 2023 17:28:39 +0000
From:   "Michael Kelley (LINUX)" <mikelley@...rosoft.com>
To:     Dexuan Cui <decui@...rosoft.com>,
        KY Srinivasan <kys@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        "wei.liu@...nel.org" <wei.liu@...nel.org>,
        "daniel.lezcano@...aro.org" <daniel.lezcano@...aro.org>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "linux-hyperv@...r.kernel.org" <linux-hyperv@...r.kernel.org>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH] clocksource: hyper-v: make sure Invariant-TSC is used if
 it is available

From: Dexuan Cui <decui@...rosoft.com> Sent: Saturday, April 8, 2023 2:04 PM
> 
> If Hyper-V TSC page is unavailable and Invariant-TSC is available,
> currently hyperv_cs_msr (rather than Invariant-TSC) is used by default.
> 
> Use Invariant-TSC by default by downgrading hyperv_cs_msr.rating in
> hv_init_tsc_clocksource(), if Invariant-TSC is available.
> 
> Signed-off-by: Dexuan Cui <decui@...rosoft.com>
> ---
>  drivers/clocksource/hyperv_timer.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/clocksource/hyperv_timer.c b/drivers/clocksource/hyperv_timer.c
> index c0cef92b12b82..7901a88ad9188 100644
> --- a/drivers/clocksource/hyperv_timer.c
> +++ b/drivers/clocksource/hyperv_timer.c
> @@ -506,9 +506,6 @@ static bool __init hv_init_tsc_clocksource(void)
>  {
>  	union hv_reference_tsc_msr tsc_msr;
> 
> -	if (!(ms_hyperv.features & HV_MSR_REFERENCE_TSC_AVAILABLE))
> -		return false;
> -
>  	/*
>  	 * If Hyper-V offers TSC_INVARIANT, then the virtualized TSC correctly
>  	 * handles frequency and offset changes due to live migration,
> @@ -525,6 +522,9 @@ static bool __init hv_init_tsc_clocksource(void)
>  		hyperv_cs_msr.rating = 250;
>  	}
> 
> +	if (!(ms_hyperv.features & HV_MSR_REFERENCE_TSC_AVAILABLE))
> +		return false;
> +
>  	hv_read_reference_counter = read_hv_clock_tsc;
> 
>  	/*
> --
> 2.25.1

Reviewed-by:  Michael Kelley <mikelley@...rosoft.com>