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.
Thanks,
M.
[0] https://lore.kernel.org/r/20210202141204.3134855-1-maz@xxxxxxxxxx
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