Re: [Qemu-devel] Migrating decrementer

[Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>]

On 03/02/16 04:59, David Gibson wrote:

>> Going back to your earlier email you suggested that the host timebase is
>> always continuously running, even when the guest is paused. But then
>> resuming the guest then the timebase must jump in the guest regardless?
>>
>> If this is the case then this is the big difference between TCG and KVM
>> guests: TCG timebase is derived from the virtual clock which solves the
>> problem of paused guests during migration. For example with the existing
>> migration code, what would happen if you did a migration with the guest
>> paused on KVM? The offset would surely be wrong as it was calculated at
>> the end of migration.
> 
> So there are two different cases to consider here.  Once is when the
> guest is paused incidentally, such as during migration, the other is
> when the guest is explicitly paused.
> 
> In the first case the timebase absolutely should keep running (or
> appear to do so), since it's the primary source of real time for the
> guest.

I'm not sure I understand this, since if the guest is paused either
deliberately or incidentally during migration then isn't the timebase
also frozen? Or is it external to the CPU?

> In the second case, it's a bit unclear what the right thing to do is.
> Keeping the tb running means accurate realtime, but stopping it is
> often better for debugging, which is one of the main reasons to
> explicitly pause.
> 
> I believe spapr on KVM HV will keep the TB going, but the TSC on x86
> will be stopped.

Is this from a guest-centric view, i.e. if I pause a VM and wait 20 mins
then when the guest resumes the timebase will jump forward by 20 mins
worth of ticks?


ATB,

Mark.