RE: [PATCH v16 0/9] Enable ptp_kvm for arm/arm64

[Jianyong Wu]

Hi Marc,

> -----Original Message-----
> From: Marc Zyngier <maz@xxxxxxxxxx>
> Sent: Tuesday, February 2, 2021 10:15 PM
> To: Jianyong Wu <Jianyong.Wu@xxxxxxx>
> Cc: netdev@xxxxxxxxxxxxxxx; yangbo.lu@xxxxxxx; john.stultz@xxxxxxxxxx;
> tglx@xxxxxxxxxxxxx; pbonzini@xxxxxxxxxx; richardcochran@xxxxxxxxx; Mark
> Rutland <Mark.Rutland@xxxxxxx>; will@xxxxxxxxxx; Suzuki Poulose
> <Suzuki.Poulose@xxxxxxx>; Andre Przywara <Andre.Przywara@xxxxxxx>;
> Steven Price <Steven.Price@xxxxxxx>; linux-kernel@xxxxxxxxxxxxxxx; linux-
> arm-kernel@xxxxxxxxxxxxxxxxxxx; kvmarm@xxxxxxxxxxxxxxxxxxxxx;
> kvm@xxxxxxxxxxxxxxx; Steve Capper <Steve.Capper@xxxxxxx>; Justin He
> <Justin.He@xxxxxxx>; nd <nd@xxxxxxx>
> 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@xxxxxxxxxx
> --
> Jazz is not dead. It just smells funny...