Re: VCPUOP_set_periodic_timer

[Andrew Cooper <andrew.cooper3@citrix.com>]

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On 15/11/13 11:24, Simon Martin wrote:
> Hi Andrew,
>  
> The operating system I am trying to port to Xen is an industrial servo
> controller. We are currently running at 125 microsecond servo update
> rate on our bespoke hardware (at the moment MIPS64 and ARM) (hence the
> 125 microsecond ideal interrupt period). We will be driving EtherCAT
> servo drives that need to be updated at 500 microseconds (hence at
> least 500 microsecond interrupt period). If we can achieve this on a
> single core ARM11 clocked at 532 MHz, it must be achievable on PC
> hardware. As I said before the idea is to dedicate a CPU core to this
> with all other functions running on the other cores.
>  
> Luckily we can accept a reasonable amount of jitter on the EtherCAT
> network as we can hand over clock synchronisation to a slave. This
> gives us a little leeway.
>  
> Regards.
>  
>  

It certainly is achievable on PC hardware, but not under the standard
assumptions in virtualisation.  You want to use
VCPUOP_set_singleshot_timer, but be aware than there are still no normal
guarantees that your domain vcpu will be run when the timer expires; the
credit scheduler will pick the highest priority vcpu to run, which might
not be the one which is expecting a timer interrupt.

You should investigate using the arinc653 scheduler in Xen, which is a
realtime scheduler, and might be more appropriate for your usecase.

~Andrew


> ------ Original Message ------
> From: "Andrew Cooper" <andrew.cooper3@citrix.com
> <mailto:andrew.cooper3@citrix.com>>
> To: "Simon Martin" <smartin@milliways.cl
> <mailto:smartin@milliways.cl>>; "xen-devel@lists.xen.org"
> <xen-devel@lists.xen.org <mailto:xen-devel@lists.xen.org>>
> Sent: 14/11/2013 18:39:21
> Subject: Re: [Xen-devel] VCPUOP_set_periodic_timer
>  
>> On 14/11/2013 21:18, Simon Martin wrote:
>>> Hi all,
>>>  
>>> I need a periodic timer running at ideally at 125 microseconds and
>>> at least 500 microseconds. I've just found the
>>> VCPUOP_set_periodic_timer, however there is a comment saying
>>> "periods less than one millisecond may not be supported".
>>>  
>>> I will be running on an x64 machine. Is this supported? If not, is
>>> there any alternate means of generating a fast interrupt?
>>>  
>>> Regards.
>>
>> What is the usecase here?  125us is very short indeed.  Xen certainly
>> cant guarantee anything more accurate than 50us.  Unless the affected
>> vcpu is running uncontested on the hardware, there is very little
>> chance that the vcpu will indeed be woken up again in 125us.
>>
>> It sounds as if you are looking for some pseudo realtime system, at
>> which point you might want to consider a different scheduler.
>>
>> ~Andrew
>>


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