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

Marcelo Tosatti
2015-01-21T14:16:48Z

On Wed, Jan 21, 2015 at 03:09:27PM +0100, Radim Krčmář wrote:
> 2015-01-20 15:54-0200, Marcelo Tosatti:
> > SuSE's 2.6.16 kernel fails to boot if the delta between tsc_timestamp
> > and rdtsc is larger than a given threshold:
> [...]
> > Disable masterclock support (which increases said delta) in case the
> > boot vcpu does not use MSR_KVM_SYSTEM_TIME_NEW.
>
> Why do we care about 2.6.16 bugs in upstream KVM?

Because people do use 2.6.16 guests.

> The code to benefit tradeoff of this patch seems bad to me ...

Can you state the tradeoff and then explain why it is bad ?

> MSR_KVM_SYSTEM_TIME is deprecated -- we could remove it now, with
> MSR_KVM_WALL_CLOCK (after hiding KVM_FEATURE_CLOCKSOURCE) if we want
> to support old guests.

What is the benefit of removing support for MSR_KVM_SYSTEM_TIME ?

Supporting old guests is important.