Re: [PATCH v19 7/7] ptp: arm/arm64: Enable ptp_kvm for arm/arm64

From: Geert Uytterhoeven <geert@linux-m68k.org>
To: Marc Zyngier <maz@kernel.org>, jianyong.wu@arm.com
Cc: netdev <netdev@vger.kernel.org>, Yangbo Lu <yangbo.lu@nxp.com>,
	John Stultz <john.stultz@linaro.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Paolo Bonzini <pbonzini@redhat.com>,
	seanjc@google.com, Richard Cochran <richardcochran@gmail.com>,
	Mark Rutland <Mark.Rutland@arm.com>,
	Will Deacon <will@kernel.org>,
	Suzuki K Poulose <suzuki.poulose@arm.com>,
	Andre Przywara <Andre.Przywara@arm.com>,
	Steven Price <steven.price@arm.com>,
	Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>,
	Sudeep Holla <sudeep.holla@arm.com>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	Linux ARM <linux-arm-kernel@lists.infradead.org>,
	kvmarm@lists.cs.columbia.edu, KVM list <kvm@vger.kernel.org>,
	Steve Capper <Steve.Capper@arm.com>,
	justin.he@arm.com, Android Kernel Team <kernel-team@android.com>
Subject: Re: [PATCH v19 7/7] ptp: arm/arm64: Enable ptp_kvm for arm/arm64
Date: Tue, 11 May 2021 11:07:59 +0200	[thread overview]
Message-ID: <CAMuHMdWd5261ti-zKsroFLvWs0abaWa7G4DKefgPwFb3rEjnNw@mail.gmail.com> (raw)
In-Reply-To: <20210330145430.996981-8-maz@kernel.org>

Hi Marc, Jianyong,

On Tue, Mar 30, 2021 at 4:56 PM Marc Zyngier <maz@kernel.org> wrote:
> From: Jianyong Wu <jianyong.wu@arm.com>
>
> Currently, there is no mechanism to keep time sync between guest and host
> in arm/arm64 virtualization environment. Time in guest will drift compared
> with host after boot up as they may both use third party time sources
> to correct their time respectively. The time deviation will be in order
> of milliseconds. But in some scenarios,like in cloud environment, we ask
> for higher time precision.
>
> kvm ptp clock, which chooses the host clock source as a reference
> clock to sync time between guest and host, has been adopted by x86
> which takes the time sync order from milliseconds to nanoseconds.
>
> This patch enables kvm ptp clock for arm/arm64 and improves clock sync precision
> significantly.

> --- a/drivers/ptp/Kconfig
> +++ b/drivers/ptp/Kconfig
> @@ -108,7 +108,7 @@ config PTP_1588_CLOCK_PCH
>  config PTP_1588_CLOCK_KVM
>         tristate "KVM virtual PTP clock"
>         depends on PTP_1588_CLOCK
> -       depends on KVM_GUEST && X86
> +       depends on (KVM_GUEST && X86) || (HAVE_ARM_SMCCC_DISCOVERY && ARM_ARCH_TIMER)

Why does this not depend on KVM_GUEST on ARM?
I.e. shouldn't the dependency be:

    KVM_GUEST && (X86 || (HAVE_ARM_SMCCC_DISCOVERY && ARM_ARCH_TIMER))

?

>         default y
>         help
>           This driver adds support for using kvm infrastructure as a PTP

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds