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Date:   Tue, 11 May 2021 10:13:03 +0100
From:   Marc Zyngier <maz@kernel.org>
To:     Geert Uytterhoeven <geert@linux-m68k.org>
Cc:     jianyong.wu@arm.com, 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
In-Reply-To: <CAMuHMdWd5261ti-zKsroFLvWs0abaWa7G4DKefgPwFb3rEjnNw@mail.gmail.com>
References: <20210330145430.996981-1-maz@kernel.org>
 <20210330145430.996981-8-maz@kernel.org>
 <CAMuHMdWd5261ti-zKsroFLvWs0abaWa7G4DKefgPwFb3rEjnNw@mail.gmail.com>
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Hi Geert,

On 2021-05-11 10:07, Geert Uytterhoeven wrote:
> 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))
> 
> ?

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.

Thanks,

         M.
-- 
Jazz is not dead. It just smells funny...