Re: HVM Migration of domU on Qemu-upstream DM causes stuck system clock with ACPI

[George Dunlap <george.dunlap@eu.citrix.com>]

On 03/06/13 18:18, Alex Bligh wrote:
> George,
>
> --On 3 June 2013 18:12:47 +0100 George Dunlap 
> <george.dunlap@eu.citrix.com> wrote:
>
>>> Will this work with all guests? We (unfortunately) do not know whether
>>> the
>>> guest is Linux with or without PV, Windows with or without PV or 
>>> what. I
>>> /think/ this is going to give problems when the guest is not using
>>> the PV timer (including Windows). Is that right?
>>>
>>> I'd be happiest if it selected native_paravirt whenever the guest was
>>> using the PV timer, but not otherwise. Happy to test patches for that.
>>
>> I'm not sure the effect, but I suspect it probably won't work quite
>> right.  Might be worth a quick test.
>>
>> We do want to fix this properly (which means, "just works w/o having to
>> use a work-around"); I'm trying to understand the exact nature of the
>> error now.
>
> Sorry did you mean using native_paravirt on Linux guests with PV support
> wouldn't work quite right, or using that on all guests wouldn't work 
> quite
> right, or that switching automatically if use of the PV timer was
> detected wouldn't work quite right?

I mean that if you enable it for guests that aren't using a PV 
clocksource (e.g., Windows guests), I'm not sure what will happen. It 
might be fine, but you should  probably test it. :-)

Switching TSC modes isn't the right thing to do -- the right thing is to 
figure out where the math is getting messed up and make it work properly 
no matter which mode you're in.

  -George