How to automate synchronizing Windows 10 guest's time with the Linux host? - kvm

Question

On Arch Linux I have a Windows 10 Guest on top of libvirt, kvm and virsh (still having some trouble to connect all these dots mentally together). Every time I suspend the laptop and a day is gone the Windows 10 host goes out of sync. I learned that with the following command I can force a time sync in the host:
➜ ~ virsh qemu-agent-command win10 '{"execute":"guest-set-time"}'
{"return":{}}
In order to make this work I modifed the clock XML block and added a kvm clock entry. This is how the block looks like now:
<clock offset="localtime">
<timer name="tsc" tickpolicy="delay"/>
<timer name="kvmclock"/>
<timer name="rtc" tickpolicy="delay" track="wall"/>
<timer name="pit" tickpolicy="delay"/>
<timer name="hpet" present="yes"/>
</clock>
I would like to know whether I can automate this step or trigger an update everytime I wake up the machine or log-in.
Thanks in advance

Answer 1

I was not able to get anywhere specifically using virsh. Here is how I fixed this issue in a Windows 11 guest on MacOS in UTM 3.6.4 and 4.1.5.
At first I tried many workarounds using w32tm - but this was always flaky.
This helped slightly:
disable "use local time for base clock" (otherwise you can't add a manual -rtc argument if using UTM)
add -rtc base=localtime,driftfix=slew
This wasn't great, because it won't recover a significant delta.
This is the solution I settled on (run in the Windows guest). It creates a scheduled task that runs every 5 minutes, gets the time from NTP, converts it to local time, measures the drift, and if the drift is >30 seconds in either direction it updates the system clock.
function Get-NtpTime
{
[OutputType([datetime])]
[CmdletBinding()]
param
(
[string]$Server = "time.nist.gov",
[int]$Port = 13
)
if (-not $PSBoundParameters.ContainsKey('ErrorAction'))
{
$ErrorActionPreference = 'Stop'
}
$Client = [Net.Sockets.TcpClient]::new($Server, $Port)
$Reader = [IO.StreamReader]::new($Client.GetStream())
try
{
$Response = $Reader.ReadToEnd()
$UtcString = $Response.Substring(7, 17)
$LocalTime = [datetime]::ParseExact(
$UtcString,
"yy-MM-dd HH:mm:ss",
[cultureinfo]::InvariantCulture,
[Globalization.DateTimeStyles]::AssumeUniversal
)
}
finally
{
$Reader.Dispose()
$Client.Dispose()
}
$LocalTime
}
function Register-TimeSync
{
[CmdletBinding()]
param
(
[Parameter()]
[timespan]$RepetitionInterval = (New-TimeSpan -Minutes 5),
[Parameter()]
[timespan]$ExecutionTimeLimit = (New-TimeSpan -Minutes 3)
)
$Invocation = {
$NtpTime = Get-NtpTime
$Delta = [datetime]::Now - $NtpTime
if ([Math]::Abs($Delta.TotalSeconds) -gt 30)
{
Set-Date $NtpTime
}
}
$PSName = if ($PSVersionTable.PSVersion.Major -le 5) {'powershell'} else {'pwsh'}
$Path = (Get-Command $PSName).Source
$Command = Get-Command Get-NtpTime
$Definition = "function Get-NtpTime`n{$($Command.Definition)}"
$Invocation = $Definition, $Invocation -join "`n"
$Bytes = [Text.Encoding]::Unicode.GetBytes($Invocation)
$Encoded = [Convert]::ToBase64String($Bytes)
$TriggerParams = #{
Once = $true
At = [datetime]::Today
RepetitionInterval = $RepetitionInterval
}
$Trigger = New-ScheduledTaskTrigger #TriggerParams
$Action = New-ScheduledTaskAction -Execute $Path -Argument "-NoProfile -EncodedCommand $Encoded"
$Settings = New-ScheduledTaskSettingsSet -ExecutionTimeLimit $ExecutionTimeLimit -MultipleInstances IgnoreNew
$Principal = New-ScheduledTaskPrincipal -UserID "NT AUTHORITY\SYSTEM" -LogonType ServiceAccount -RunLevel Highest
$RegisterParams = #{
TaskName = "Update system time from NTP"
Trigger = $Trigger
Action = $Action
Settings = $Settings
Principal = $Principal
Force = $true
}
Register-ScheduledTask #RegisterParams
}
Usage (run as admin):
Register-TimeSync