linux-kernel - Re: [RFC 2/2] x86/kvmclock: Use host timekeeping.

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Date:   Fri, 20 Sep 2019 15:33:47 +0200
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Suleiman Souhlal <suleiman@...gle.com>
Cc:     john.stultz@...aro.org, sboyd@...nel.org,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        Suleiman Souhlal <suleiman@...gle.com>, pbonzini@...hat.com,
        rkrcmar@...hat.com, tglx@...utronix.de
Subject: Re: [RFC 2/2] x86/kvmclock: Use host timekeeping.

Suleiman Souhlal <suleiman@...gle.com> writes:

> When CONFIG_KVMCLOCK_HOST_TIMEKEEPING is enabled, and the host
> supports it, update our timekeeping parameters to be the same as
> the host. This lets us have our time synchronized with the host's,
> even in the presence of host NTP or suspend.
>
> When enabled, kvmclock uses raw tsc instead of pvclock.
>
> When enabled, syscalls that can change time, such as settimeofday(2)
> or adj_timex(2) are disabled in the guest.
>
> Signed-off-by: Suleiman Souhlal <suleiman@...gle.com>
> ---
>  arch/x86/Kconfig                |   9 +++
>  arch/x86/include/asm/kvmclock.h |   2 +
>  arch/x86/kernel/kvmclock.c      | 127 +++++++++++++++++++++++++++++++-
>  kernel/time/timekeeping.c       |  21 ++++++
>  4 files changed, 155 insertions(+), 4 deletions(-)
>
> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
> index 4195f44c6a09..37299377d9d7 100644
> --- a/arch/x86/Kconfig
> +++ b/arch/x86/Kconfig
> @@ -837,6 +837,15 @@ config PARAVIRT_TIME_ACCOUNTING
>  config PARAVIRT_CLOCK
>  	bool
>  
> +config KVMCLOCK_HOST_TIMEKEEPING
> +	bool "kvmclock uses host timekeeping"
> +	depends on KVM_GUEST
> +	---help---
> +	  Select this option to make the guest use the same timekeeping
> +	  parameters as the host. This means that time will be almost
> +	  exactly the same between the two. Only works if the host uses "tsc"
> +	  clocksource.
> +

I'd also like to speak up against this config, it is confusing. In case
the goal is to come up with a TSC-based clock for guests which will
return the same as clock_gettime() on the host (or, is the goal to just
have the same reading for all guests on the host?) I'd suggest we create
a separate (from KVMCLOCK) clocksource (mirroring host timekeeper) and
guests will be free to pick the one they like.

-- 
Vitaly