linux-kernel - RE: [kvm-devel] [PATCH 2/3] kvmclock

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Date:	Tue, 13 Nov 2007 23:23:49 +0800
From:	"Dong, Eddie" <eddie.dong@...el.com>
To:	"Avi Kivity" <avi@...ranet.com>,
	"Glauber de Oliveira Costa" <gcosta@...hat.com>
Cc:	<linux-kernel@...r.kernel.org>, <jeremy@...p.org>,
	<hollisb@...ibm.com>, <kvm-devel@...ts.sourceforge.net>
Subject: RE: [kvm-devel] [PATCH 2/3] kvmclock - the host part.

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
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