Re: [PATCH] qemu-kvm: introduce cpu_start/cpu_stop commands

[Anthony Liguori <aliguori@linux.vnet.ibm.com>]

On 11/22/2010 05:56 PM, Chris Wright wrote:
> * Anthony Liguori (aliguori@linux.vnet.ibm.com) wrote:
>    
>> On 11/22/2010 05:04 PM, Chris Wright wrote:
>>      
>>> * Anthony Liguori (aliguori@us.ibm.com) wrote:
>>>        
>>>> qemu-kvm vcpu threads don't response to SIGSTOP/SIGCONT.  Instead of teaching
>>>> them to respond to these signals, introduce monitor commands that stop and start
>>>> individual vcpus.
>>>>          
>>> In the past SIGSTOP has introduced time skew.  Have you verified this
>>> isn't an issue.
>>>        
>> Time skew is a big topic.  Are you talking about TSC drift,
>> pit/rtc/hpet drift, etc?
>>      
> Sorry to be vague, but it's been long enough that I don't recall
> the details.  The guest kernel's clocksource effected how timekeeping
> progressed across STOP/CONT (was probably missing qemu based timer ticks).
> While this is not the same, made me wonder if you'd tested against that.
>    

Yeah, it's definitely going to increase the likelihood of interrupt 
coalescing but only as much as a contended CPU would already.

QEMU will keep getting timer ticks but the guest won't process them in a 
timely fashion.

>> It's certainly going to stress periodic interrupt catch up code.
>>      
> Heh, call it a feature for autotest ;)
>    

Excellent idea :-)

Regards,

Anthony Liguori

> thanks,
> -chris
>