linux-kernel - Re: [PATCH v19 7/7] ptp: arm/arm64: Enable ptp

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Date:   Tue, 11 May 2021 11:07:59 +0200
From:   Geert Uytterhoeven <geert@...ux-m68k.org>
To:     Marc Zyngier <maz@...nel.org>, jianyong.wu@....com
Cc:     netdev <netdev@...r.kernel.org>, Yangbo Lu <yangbo.lu@....com>,
        John Stultz <john.stultz@...aro.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Paolo Bonzini <pbonzini@...hat.com>, seanjc@...gle.com,
        Richard Cochran <richardcochran@...il.com>,
        Mark Rutland <Mark.Rutland@....com>,
        Will Deacon <will@...nel.org>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Andre Przywara <Andre.Przywara@....com>,
        Steven Price <steven.price@....com>,
        Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
        Sudeep Holla <sudeep.holla@....com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        kvmarm@...ts.cs.columbia.edu, KVM list <kvm@...r.kernel.org>,
        Steve Capper <Steve.Capper@....com>, justin.he@....com,
        Android Kernel Team <kernel-team@...roid.com>
Subject: Re: [PATCH v19 7/7] ptp: arm/arm64: Enable ptp_kvm for arm/arm64

Hi Marc, Jianyong,

On Tue, Mar 30, 2021 at 4:56 PM Marc Zyngier <maz@...nel.org> wrote:
> From: Jianyong Wu <jianyong.wu@....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@...ux-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