Re: [Qemu-devel] invtsc + migration + TSC scaling

[Radim Krčmář]

2016-10-17 21:58-0200, Marcelo Tosatti:
> On Mon, Oct 17, 2016 at 07:11:01PM -0200, Eduardo Habkost wrote:
>> On Mon, Oct 17, 2016 at 06:24:38PM +0200, Paolo Bonzini wrote:
>> > On 17/10/2016 16:50, Radim Krčmář wrote:
>> > > 2016-10-17 07:47-0200, Marcelo Tosatti:
>> [...]
>> > >> since Linux guests use kvmclock and Windows guests use Hyper-V
>> > >> enlightenment, it should be fine to disable 2).
>> > 
>> > ... and 1 too.
>> > 
>> > We should also blacklist the TSC deadline timer when invtsc is not
>> > available.
> 
> Actually, a nicer fix would be to check the different 
> frequencies and scale the deadline relative to the difference. 

I think that KVM can already be configured to do that.

Paolo, we hit that TSC deadline bug bacause QEMU doesn't set the TSC
frequency if it would result in software scaling (which needs to update
guest TSC and kvmclock on every entry)?

Thanks.

(I just noticed a minor bug: KVM doesn't use hardware scaling when the
 TSC frequency delta is small.)

> This would take care of both patched and non-patched guests.
> 
> On a related note, what was the goal of Radim's paravirtual deadline
> TSC timer?

It's be paravirtual kvmclock timer -- just giving the deadline in other
another time frame.  It won't confuse OS that expect the deadline timer
to behave like it should. :)