linux-kernel - RE: [PATCH v16 0/9] Enable ptp

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Date:   Wed, 3 Feb 2021 02:40:01 +0000
From:   Jianyong Wu <Jianyong.Wu@....com>
To:     Marc Zyngier <maz@...nel.org>
CC:     "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "yangbo.lu@....com" <yangbo.lu@....com>,
        "john.stultz@...aro.org" <john.stultz@...aro.org>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "richardcochran@...il.com" <richardcochran@...il.com>,
        Mark Rutland <Mark.Rutland@....com>,
        "will@...nel.org" <will@...nel.org>,
        Suzuki Poulose <Suzuki.Poulose@....com>,
        Andre Przywara <Andre.Przywara@....com>,
        Steven Price <Steven.Price@....com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "kvmarm@...ts.cs.columbia.edu" <kvmarm@...ts.cs.columbia.edu>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        Steve Capper <Steve.Capper@....com>,
        Justin He <Justin.He@....com>, nd <nd@....com>
Subject: RE: [PATCH v16 0/9] Enable ptp_kvm for arm/arm64

Hi Marc,

> -----Original Message-----
> From: Marc Zyngier <maz@...nel.org>
> Sent: Tuesday, February 2, 2021 10:15 PM
> To: Jianyong Wu <Jianyong.Wu@....com>
> Cc: netdev@...r.kernel.org; yangbo.lu@....com; john.stultz@...aro.org;
> tglx@...utronix.de; pbonzini@...hat.com; richardcochran@...il.com; Mark
> Rutland <Mark.Rutland@....com>; will@...nel.org; Suzuki Poulose
> <Suzuki.Poulose@....com>; Andre Przywara <Andre.Przywara@....com>;
> Steven Price <Steven.Price@....com>; linux-kernel@...r.kernel.org; linux-
> arm-kernel@...ts.infradead.org; kvmarm@...ts.cs.columbia.edu;
> kvm@...r.kernel.org; Steve Capper <Steve.Capper@....com>; Justin He
> <Justin.He@....com>; nd <nd@....com>
> Subject: Re: [PATCH v16 0/9] Enable ptp_kvm for arm/arm64
> 
> On 2020-12-09 06:09, Jianyong Wu wrote:
> > Currently, we offen use ntp (sync time with remote network clock) to
> > sync time in VM. But the precision of ntp is subject to network delay
> > so it's difficult to sync time in a high precision.
> >
> > kvm virtual ptp clock (ptp_kvm) offers another way to sync time in VM,
> > as the remote clock locates in the host instead of remote network
> > clock.
> > It targets to sync time between guest and host in virtualization
> > environment and in this way, we can keep the time of all the VMs
> > running in the same host in sync. In general, the delay of
> > communication between host and guest is quiet small, so ptp_kvm can
> > offer time sync precision up to in order of nanosecond. Please keep in
> > mind that ptp_kvm just limits itself to be a channel which transmit
> > the remote clock from host to guest and leaves the time sync jobs to
> > an application, eg.
> > chrony,
> > in usersapce in VM.
> >
> > How ptp_kvm works:
> > After ptp_kvm initialized, there will be a new device node under /dev
> > called ptp%d. A guest userspace service, like chrony, can use this
> > device to get host walltime, sometimes also counter cycle, which
> > depends on the service it calls. Then this guest userspace service can
> > use those data to do the time sync for guest.
> > here is a rough sketch to show how kvm ptp clock works.
> >
> > |----------------------------|
> > |--------------------------|
> > |       guest userspace      |              |          host
> > |
> > |ioctl -> /dev/ptp%d         |              |
> > |
> > |       ^   |                |              |
> > |
> > |----------------------------|              |
> > |
> > |       |   | guest kernel   |              |
> > |
> > |       |   V      (get host walltime/counter cycle)
> > |
> > |      ptp_kvm -> hypercall - - - - - - - - - - ->hypercall service
> > |
> > |                         <- - - - - - - - - - - -
> > |
> > |----------------------------|
> > |--------------------------|
> >
> > 1. time sync service in guest userspace call ptp device through
> > /dev/ptp%d.
> > 2. ptp_kvm module in guest receives this request then invoke hypercall
> > to route into host kernel to request host walltime/counter cycle.
> > 3. ptp_kvm hypercall service in host response to the request and send
> > data back.
> > 4. ptp (not ptp_kvm) in guest copy the data to userspace.
> >
> > This ptp_kvm implementation focuses itself to step 2 and 3 and step 2
> > works in guest comparing step 3 works in host kernel.
> 
> FWIW, and in order to speed up the review, I've posted a reworked
> version[0] of this series with changes that address the comments I had for on
> v16.
> 

Great!!!
Good news for me, thanks Marc.

Thanks
Jianyong

> Thanks,
> 
>          M.
> 
> [0] https://lore.kernel.org/r/20210202141204.3134855-1-maz@kernel.org
> --
> Jazz is not dead. It just smells funny...