Re: [kvm-devel] [PATCH 2/3] kvmclock - the host part.

[Avi Kivity <avi@qumranet.com>]

Dong, Eddie wrote:
> > > After thinking for a little while, you are theoretically right.
> > > In the current state, we could even be preempted between all
> > > operations ;-) Maybe after avi's suggestion of moving the call to it
> > > it will end up in a preempt safe region, but anyway, it's safer to
> > > add the preempt markers here. I'll put it in next version, thanks
> > >
> > >
> > >       
> > Well, you can't kvm_write_guest() with preemption enabled.
> >
> > preempt notifiers to the rescue!  We have a callout during preemption,
> > so you can just zero out a flag there, and when we're scheduled again
> > retry the whole thing. 
> >
> >     
>
> The preemption issue is within following code which need to be done in a
> short enough period.
>
> +	kvm_get_msr(vcpu, MSR_IA32_TIME_STAMP_COUNTER,
> +			  &vcpu->hv_clock.last_tsc);
> +
> +	ktime_get_ts(&ts);
> +	vcpu->hv_clock.now_ns = ts.tv_nsec + (NSEC_PER_SEC *
> (u64)ts.tv_sec);
> +	vcpu->hv_clock.wc_sec = get_seconds();
>
> I am even thinking we have to disable interrupt between these lines,
> otherwise
> guest wall clock may see backward time source when calculating the
> delta TSC since last vcpu->hv_clock.now_ns update.
>   

That's true.  While we do need to handle vcpu migration and 
descheduling, the code sequence you note needs to be as atomic as possible.


--
error compiling committee.c: too many arguments to function

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