linux-kernel - Re: [PATCH v5 05/16] clocksource/hyperv: use MSR-based access if running as root

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Message-ID: <CA+CK2bBTjUWEOrFKi4pYpEe355sve6b7AjKGc7cQRRe3c-DTrQ@mail.gmail.com>
Date:   Wed, 20 Jan 2021 11:13:28 -0500
From:   Pavel Tatashin <pasha.tatashin@...een.com>
To:     Wei Liu <wei.liu@...nel.org>
Cc:     Linux on Hyper-V List <linux-hyperv@...r.kernel.org>,
        virtualization@...ts.linux-foundation.org,
        Linux Kernel List <linux-kernel@...r.kernel.org>,
        Michael Kelley <mikelley@...rosoft.com>,
        Vineeth Pillai <viremana@...ux.microsoft.com>,
        Sunil Muthuswamy <sunilmut@...rosoft.com>,
        Nuno Das Neves <nunodasneves@...ux.microsoft.com>,
        Daniel Lezcano <daniel.lezcano@...aro.org>,
        "K. Y. Srinivasan" <kys@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH v5 05/16] clocksource/hyperv: use MSR-based access if
 running as root

On Wed, Jan 20, 2021 at 7:01 AM Wei Liu <wei.liu@...nel.org> wrote:
>
> When Linux runs as the root partition, the setup required for TSC page
> is different.

Why would we need a TSC page as a clock source for root partition at
all? I think the above can be removed.

 Luckily Linux also has access to the MSR based
> clocksource. We can just disable the TSC page clocksource if Linux is
> the root partition.
>
> Signed-off-by: Wei Liu <wei.liu@...nel.org>
> Acked-by: Daniel Lezcano <daniel.lezcano@...aro.org>
> ---
>  drivers/clocksource/hyperv_timer.c | 3 +++
>  1 file changed, 3 insertions(+)
>
> diff --git a/drivers/clocksource/hyperv_timer.c b/drivers/clocksource/hyperv_timer.c
> index ba04cb381cd3..269a691bd2c4 100644
> --- a/drivers/clocksource/hyperv_timer.c
> +++ b/drivers/clocksource/hyperv_timer.c
> @@ -426,6 +426,9 @@ static bool __init hv_init_tsc_clocksource(void)
>         if (!(ms_hyperv.features & HV_MSR_REFERENCE_TSC_AVAILABLE))
>                 return false;
>
> +       if (hv_root_partition)
> +               return false;
> +

Reviewed-by: Pavel Tatashin <pasha.tatashin@...een.com>