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

[Marc Zyngier]

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@xxxxxxxxxx> wrote:
> > From: Jianyong Wu <jianyong.wu@xxxxxxx>
> >
> > 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.
--
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