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From: "Dong, Eddie" <eddie.dong@intel.com>
To: "Avi Kivity" <avi@qumranet.com>,
	"Glauber de Oliveira Costa" <gcosta@redhat.com>
Cc: <linux-kernel@vger.kernel.org>, <jeremy@goop.org>,
	<hollisb@us.ibm.com>, <kvm-devel@lists.sourceforge.net>
Subject: RE: [kvm-devel] [PATCH 2/3] kvmclock - the host part.
Date: Tue, 13 Nov 2007 23:23:49 +0800	[thread overview]
Message-ID: <10EA09EFD8728347A513008B6B0DA77A025DFB75@pdsmsx411.ccr.corp.intel.com> (raw)
In-Reply-To: <4739B916.4000405@qumranet.com>

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

  reply	other threads:[~2007-11-13 15:24 UTC|newest]

Thread overview: 16+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2007-11-08 22:39 [PATCH 0/3] Kvm clocksource, new spin Glauber de Oliveira Costa
2007-11-08 22:39 ` [PATCH 1/3] include files for kvmclock Glauber de Oliveira Costa
2007-11-08 22:39   ` [PATCH 2/3] kvmclock - the host part Glauber de Oliveira Costa
2007-11-08 22:39     ` [PATCH 3/3] kvmclock implementation, the guest part Glauber de Oliveira Costa
2007-11-11 10:17     ` [PATCH 2/3] kvmclock - the host part Avi Kivity
2007-11-13 11:28       ` Glauber de Oliveira Costa
2007-11-13 14:44         ` Avi Kivity
2007-11-13  5:00     ` [kvm-devel] " Dong, Eddie
2007-11-13 11:54       ` Glauber de Oliveira Costa
2007-11-13 12:08         ` Izik Eidus
2007-11-13 14:47         ` Avi Kivity
2007-11-13 15:23           ` Dong, Eddie [this message]
2007-11-13 16:12             ` Avi Kivity
2007-11-14  0:41               ` Dong, Eddie
2007-11-09  8:37   ` [PATCH 1/3] include files for kvmclock Gerd Hoffmann
2007-11-11  9:15     ` Avi Kivity

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