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

Dong, Eddie
2007-11-13T15:24:13Z

Avi Kivity wrote:
> Glauber de Oliveira Costa wrote:
>> -----BEGIN PGP SIGNED MESSAGE-----
>> Hash: SHA1
>>
>> Dong, Eddie escreveu:
>>
>>>> +static void kvm_write_guest_time(struct kvm_vcpu *vcpu) +{
>>>> + struct timespec ts; + int r;
>>>> +
>>>> + if (!vcpu->clock_gpa)
>>>> + return;
>>>> +
>>>> + /* Updates version to the next odd number, indicating we're
>>>> writing */ + vcpu->hv_clock.version++;
>>>> + kvm_write_guest(vcpu->kvm, vcpu->clock_gpa,
>>>> &vcpu->hv_clock, PAGE_SIZE);
>>>> +
>>>> + kvm_get_msr(vcpu, MSR_IA32_TIME_STAMP_COUNTER,
>>>> + &vcpu->hv_clock.last_tsc);
>>>> +
>>>> + ktime_get_ts(&ts);
>>>>
>>> Do we need to disable preemption here?
>>>
>> 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.

Also why we use both ktime_get_ts(&ts) & get_seconds() together? they
are not atomic
and may cause issues?

thx,eddie