DomU to Dom0 Communication in Xen HVM?

DomU to Dom0 Communication in Xen HVM?

[Larry White]

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I am new to Xen and want to provide simple communication between my DomU and
Dom0 environments.  I just need to be able to pass a few bytes of
information back and forth between DomU and Dom0 while using some form of
event management to know when an update occurs so I can act on it.  At
first, it appeared that using XenBus and XenStore might be the right
solution for this.  However, after looking over the documentation, it seems
that XenBus and XenStore should only be used in a PVM Xen configuration, not
a HVM model like mine.  Is this true?  

 

What would be the best way to accomplish this?  Is there any sample code
which demonstrates this for a HVM Xen environment?

 

Any information regarding this would be greatly apprecated.

 

Larry

 


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Re: DomU to Dom0 Communication in Xen HVM?

[Konrad Rzeszutek Wilk]

On Tue, Jul 17, 2012 at 11:22:28PM -0500, Larry White wrote:
> I am new to Xen and want to provide simple communication between my DomU and
> Dom0 environments.  I just need to be able to pass a few bytes of
> information back and forth between DomU and Dom0 while using some form of
> event management to know when an update occurs so I can act on it.  At
> first, it appeared that using XenBus and XenStore might be the right
> solution for this.  However, after looking over the documentation, it seems
> that XenBus and XenStore should only be used in a PVM Xen configuration, not
> a HVM model like mine.  Is this true?  

It can be done in HVM.
> 
>  
> 
> What would be the best way to accomplish this?  Is there any sample code
> which demonstrates this for a HVM Xen environment?

libvchan is your best bet.
> 
>  
> 
> Any information regarding this would be greatly apprecated.
> 
>  
> 
> Larry
> 
>  
> 

> _______________________________________________
> Xen-devel mailing list
> Xen-devel@lists.xen.org
> http://lists.xen.org/xen-devel

XenStore as a data transfer path?

[Larry White]

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Can the XenStore database be accessed by domU's?  

 

If so, could it be used to pass small amounts of data back and forth between
dom0 and domU's using XenStore key entries?  If this is possible, it would
seem that this could eliminate the need to create a "device" driver for a
simple communication path.

 

Also, what type of Guest Fedora Kernel would be needed to perform this type
of Xen functionality in a HVM domU guest configuration?  I assume that a
normal Fedora kernel would not work.  However, I have read the Fedora 14 and
later has some type of Xen support out of the box.  Not sure what type of
support.

 

Thanks,

Larry

 


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Re: XenStore as a data transfer path?

[James Harper]

> 
> Can the XenStore database be accessed by domU's?
> 

Yes. That is how the frontend/backend devices set up their communication channels.

> 
> If so, could it be used to pass small amounts of data back and forth between
> dom0 and domU's using XenStore key entries?  If this is possible, it would
> seem that this could eliminate the need to create a "device" driver for a
> simple communication path.
> 

Yes, for certain values of "small". Can you say more about the sort of data you would be passing? In particular the throughput and latency requirements?

James

Re: XenStore as a data transfer path?

[Larry White]

-----Original Message-----
From: James Harper [mailto:james.harper@bendigoit.com.au] 
Sent: Friday, July 20, 2012 8:44 PM
To: Larry White; xen-devel@lists.xen.org
Subject: RE: [Xen-devel] XenStore as a data transfer path?

> 
> Can the XenStore database be accessed by domU's?
> 

>>Yes. That is how the frontend/backend devices set up their communication
>>channels.

Are you saying that the actual data going back and forth gets put into the
XenStore?  I thought that the XenStore was mostly used for configuration
keyword values and such that the underlying drivers needed.

> 
> If so, could it be used to pass small amounts of data back and forth
between
> dom0 and domU's using XenStore key entries?  If this is possible, it would
> seem that this could eliminate the need to create a "device" driver for a
> simple communication path.
> 

>>Yes, for certain values of "small". Can you say more about the sort of
>>data you would be passing? In particular the throughput and latency
>>requirements?

I would say that the small amount of data here per transfer would be around
32 bytes in length with a latency of around 500ms or so.

Larry

Re: XenStore as a data transfer path?

[James Harper]

> >
> > Can the XenStore database be accessed by domU's?
> >
> 
> >>Yes. That is how the frontend/backend devices set up their
> >>communication channels.
> 
> Are you saying that the actual data going back and forth gets put into the
> XenStore?  I thought that the XenStore was mostly used for configuration
> keyword values and such that the underlying drivers needed.
> 

Yes. It's how they set up their communication channels. The communication channels are shared pages of memory and event channels, but they use xenstore to set it up (eg so the backend can find the address of the frontend ring.

> >
> > If so, could it be used to pass small amounts of data back and forth
> between
> > dom0 and domU's using XenStore key entries?  If this is possible, it
> > would seem that this could eliminate the need to create a "device"
> > driver for a simple communication path.
> >
> 
> >>Yes, for certain values of "small". Can you say more about the sort of
> >>data you would be passing? In particular the throughput and latency
> >>requirements?
> 
> I would say that the small amount of data here per transfer would be around
> 32 bytes in length with a latency of around 500ms or so.
> 

How fast though? Is it 32 bytes every 1ms, or 32 bytes every 20 seconds?

James

Re: DomU to Dom0 Communication in Xen HVM?

[Larry White]

Hi Konrad,

>On Tue, Jul 17, 2012 at 11:22:28PM -0500, LarryWhite wrote:
>I am new to Xen and want to provide simple communication between my DomU
>and
>Dom0 environments.  I just need to be able to pass a few bytes of
>information back and forth between DomU and Dom0 while using some form of
>event management to know when an update occurs so I can act on it.  At
>first, it appeared that using XenBus and XenStore might be the right
>solution for this.  However, after looking over the documentation, it seems
>that XenBus and XenStore should only be used in a PVM Xen configuration,
>not
>a HVM model like mine.  Is this true?  

>>It can be done in HVM.

>What would be the best way to accomplish this?  Is there any sample code
>which demonstrates this for a HVM Xen environment?

>>libvchan is your best bet.

Have been looking at libvchan as you suggested.  An additional bit of
information that I left out on my original post was that I would like to do
this in a "one to many" environment.  In other words, I want to support data
transfer from dom0 to a number of different domU's.  Would an approach such
as libvchan allow for this or would I need a separate process in dom0 for
each of the different domU channels?  I was really hoping to have a single
process running in dom0 to manage this. 

Thanks,
Larry