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>
Cc: KVM list <kvm@vger.kernel.org>,
	seanjc@google.com, Will Deacon <will@kernel.org>,
	kvmarm@lists.cs.columbia.edu,
	Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>,
	justin.he@arm.com, Steven Price <steven.price@arm.com>,
	Android Kernel Team <kernel-team@android.com>,
	Andre Przywara <Andre.Przywara@arm.com>,
	Richard Cochran <richardcochran@gmail.com>,
	Yangbo Lu <yangbo.lu@nxp.com>,
	John Stultz <john.stultz@linaro.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Linux ARM <linux-arm-kernel@lists.infradead.org>,
	netdev <netdev@vger.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	Sudeep Holla <sudeep.holla@arm.com>,
	Paolo Bonzini <pbonzini@redhat.com>
Subject: Re: [PATCH v19 7/7] ptp: arm/arm64: Enable ptp_kvm for arm/arm64
Date: Wed, 26 May 2021 09:52:42 +0200	[thread overview]
Message-ID: <CAMuHMdWzBqLVOVn_z8S2H-x-kL+DfOsM5mDb_D8OKsyRJtKpdA@mail.gmail.com> (raw)
In-Reply-To: <6c522f8116f54fa6f23a2d217d966c5a@kernel.org>

Hi Marc,

On Tue, May 11, 2021 at 11:13 AM Marc Zyngier <maz@kernel.org> wrote:
> On 2021-05-11 10:07, Geert Uytterhoeven wrote:
> > 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))
> >
> > ?
>
> arm/arm64 do not select KVM_GUEST. Any kernel can be used for a guest,
> and KVM/arm64 doesn't know about this configuration symbol.

OK.

Does PTP_1588_CLOCK_KVM need to default to yes?
Perhaps only on X86, to maintain the status quo?

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
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