Re: KVM: x86: workaround SuSE's 2.6.16 pvclock vs masterclock issue

Radim Krčmář
2015-01-22T19:33:35Z

2015-01-22 16:12-0200, Marcelo Tosatti:
> On Thu, Jan 22, 2015 at 02:59:28PM +0100, Radim Krčmář wrote:
> > I don't understand what the guest wants to achieve with the delta.
>
> Neither do I. It seems to assume that TSC delta is unreliable, while
> system_timestamp is reliable.
>
> Therefore if TSC delta is large, request a system_timestamp update,
> which keeps TSC delta small.

I was wondering how they do that ... kvm clock has like 1 MSR interface;
multiple registrations are supposed to work?

> > I thought that checking this only made sense if the guest didn't believe
> > that PV clock works with large delta. And they only want precision.
> > (What else is there on a clock?)
>
> Ok right they assumed TSC was not reliable?

Most likely. (And that doing complicated magic is going to help it --
doesn't VMENTRY do pretty much everyting that can be done with
unreliable TSC?)