Clock running at double speed in guest

Clock running at double speed in guest

[Tim Chen]

I notice that when I am testing kvm, the clock on guest is running at
twice the speed.  Same thing also happen using qemu without kvm.

I look at my wall clock when I issue the command "time sleep 30".  When
the command returns and report 30 sec has elasped, I found that actually
only 15 sec has passed on the wall clock.  Perhaps somewhere an extra
clock tick was counted.

I am using 2.6.20-rc5 on host and 2.6.19 on guest.  Test is done on a
dual socket woodcrest machine.

Tim

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Re: Clock running at double speed in guest

[Avi Kivity]

Tim Chen wrote:
> I notice that when I am testing kvm, the clock on guest is running at
> twice the speed.  Same thing also happen using qemu without kvm.
>
> I look at my wall clock when I issue the command "time sleep 30".  When
> the command returns and report 30 sec has elasped, I found that actually
> only 15 sec has passed on the wall clock.  Perhaps somewhere an extra
> clock tick was counted.
>
>   

That's a known issue, I plan to take a look at it soon.


-- 
Do not meddle in the internals of kernels, for they are subtle and quick to panic.


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Re: Clock running at double speed in guest

[Dor Laor]

>Tim Chen wrote:
>> I notice that when I am testing kvm, the clock on guest is running at
>> twice the speed.  Same thing also happen using qemu without kvm.
>>
>> I look at my wall clock when I issue the command "time sleep 30".
When
>> the command returns and report 30 sec has elasped, I found that
actually
>> only 15 sec has passed on the wall clock.  Perhaps somewhere an extra
>> clock tick was counted.
>>
>>
>
>That's a known issue, I plan to take a look at it soon.
>

Mean while try clock=pit in the guest's kernel command line.

>
>--
>Do not meddle in the internals of kernels, for they are subtle and
quick to
>panic.
>
>
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Re: Clock running at double speed in guest

[Tim Chen]

On Sun, 2007-01-28 at 01:37 -0800, Dor Laor wrote:

> 
> Mean while try clock=pit in the guest's kernel command line.
> 

Mmmm..., this does not fix the problem.  So perhaps it is not due to
missing clock ticks.

Tim 

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Re: Clock running at double speed in guest

[Avi Kivity]

Tim Chen wrote:
> I notice that when I am testing kvm, the clock on guest is running at
> twice the speed.  Same thing also happen using qemu without kvm.
>
> I look at my wall clock when I issue the command "time sleep 30".  When
> the command returns and report 30 sec has elasped, I found that actually
> only 15 sec has passed on the wall clock.  Perhaps somewhere an extra
> clock tick was counted.
>
> I am using 2.6.20-rc5 on host and 2.6.19 on guest.  Test is done on a
> dual socket woodcrest machine.
>
>   

I've managed to reproduce this briefly.  As soon as I started debugging 
it, however, it went away.

Does this reproduce regularly for you?  Do you run kvm as a user that 
has access to /dev/rtc?

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


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Re: Clock running at double speed in guest

[Tim Chen]

On Tue, 2007-03-06 at 15:34 +0200, Avi Kivity wrote:
>   
> 
> I've managed to reproduce this briefly.  As soon as I started debugging 
> it, however, it went away.
> Does this reproduce regularly for you?  

This is produced consistently in my environment.

> Do you run kvm as a user that 
> has access to /dev/rtc?
> 

I haven't done anything specifically with respect to /dev/rtc.  How do
you tell if kvm has access to /dev/rtc?

Tim

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Re: Clock running at double speed in guest

[Avi Kivity]

Tim Chen wrote:
>   
>> Do you run kvm as a user that 
>> has access to /dev/rtc?
>>
>>     
>
> I haven't done anything specifically with respect to /dev/rtc.  How do
> you tell if kvm has access to /dev/rtc?
>
>   

The normal unix permissions on /dev/rtc.  If you're running as root, you 
have them.


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


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Re: Clock running at double speed in guest

[Tim Chen]

On Tue, 2007-03-06 at 18:52 +0200, Avi Kivity wrote:

> The normal unix permissions on /dev/rtc.  If you're running as root, you 
> have them.
> 
> 

Yes, I'm running as root.

Tim

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Re: Clock running at double speed in guest

[Dor Laor]

>>
>>
>> I've managed to reproduce this briefly.  As soon as I started
debugging
>> it, however, it went away.
>> Does this reproduce regularly for you?
>
>This is produced consistently in my environment.
>
>> Do you run kvm as a user that
>> has access to /dev/rtc?
>>
>
>I haven't done anything specifically with respect to /dev/rtc.  How do
>you tell if kvm has access to /dev/rtc?


ls -la /proc/`pgrep qemu`/fd | grep rtc

>
>Tim
>
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Re: Clock running at double speed in guest

[Tim Chen]

> >I haven't done anything specifically with respect to /dev/rtc.  How do
> >you tell if kvm has access to /dev/rtc?
> 
> 
> ls -la /proc/`pgrep qemu`/fd | grep rtc
> 

I got: 

lr-x------ 1 root root 64 Mar  6 10:12 4 -> /dev/rtc

indicating that qemu has access to /dev/rtc.

Tim

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