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Re: [Xen-devel] questions regarding interdomain communication and xen-guest communication

On 24 October 2013 09:09, Jan Beulich <jbeulich@xxxxxxxx> wrote:
>>>> Aastha Mehta <aasthakm@xxxxxxxxx> 10/23/13 7:44 PM >>>
>>If there is an interdomain event channel set up between two guests, do
>>they communicate directly with each other, bypassing the hypervisor
> No, event channel delivery works via the hypervisor.
>> In case of an asynchronous I/O does one guest have to
>>register its callbacks with the other guest directly?
>>Or does one guest hypercall into xen, which then upcalls into the
>>other guest and then transfer reply in the reverse using another pair
>>of hypercall and upcall?
> Again, the latter.

So when one guest (G1) shares pages with second (G2), for example for
block I/O, G1 will inform second via xen about a shared page. G1 will
also register a callback with Xen. When Xen upcalls into G2 to forward
G1's request, does it block on G2's reponse? How does G2 send back the
result of I/O to G1?

>> Can event channels be set up ad hoc between
>>two, for example, dom0 and a guest domain? Or can they only be used by
>>drivers, e.g. between network frontend and backend drivers?
> The former.
>>Regarding xen -- domain communication: how can xen send some request
>>to dom0 for some operation that needs to be performed in the dom0 and
>>get back results from the dom0?
> Look at the MCE polling Dom0 performs for example, or the memory pressure
> threshold handling: One model would be for Dom0 to set up a (dedicated)
> vIRQ for that purpose, and Xen trigger that vIRQ when it needs Dom0's help.

Just to clarify, will guest return the result of the operation using a

>> Does such a communication have to be synchronous?
> I would say no, unless your use case has such a requirement.
> Jan

Thanks for the response.

Aastha Mehta
MPI-SWS, Germany
E-mail: aasthakm@xxxxxxxxxxx

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