Re: [Qemu-devel] [PATCH 4/4] pl031: switch clock base to rtc_clock

[Paolo Bonzini <pbonzini@redhat.com>]

On 01/30/2012 05:54 PM, Peter Maydell wrote:
>> >  This lets the user specify the desired semantics.  By default, the RTC
>> >  will follow adjustments from the host's NTP client, and will remain in
>> >  sync when the virtual machine is stopped.  The previous behavior, which
>> >  provides determinism with both icount and qtest, remains available with
>> >  "-rtc clock=vm".
>> >
>> >  pl031 supports migration, so we need to convert the time
>> >  base from rtc_clock to vm_clock and back for backwards compatibility.
> If it would be cleaner I'm entirely happy for us to do a version
> bump and break binary compatibility. (We bump the version on
> the CPUState pretty regularly anyhow; save/load for ARM devices
> currently is mostly for the benefit of snapshot save/restore rather
> than between-qemu-versions migration.)

Actually it's not really just backwards compatibility.

rtc_clock may be counting up from a different time base on the source 
and destination machine.  vm_clock is the only time base that is 
migrated, so it is the only one that is safe to use in the data stream.

Perhaps I could transfer the timer deadline relative to the current 
rtc_clock value and convert it again on the destination.  However, it 
would be basically the same code that transfers the value relative to 
the vm_clock; it's just a different offset to apply.

Paolo