Re: [PATCH] xen/evtchn: Add design for static event channel signaling for domUs..

[Rahul Singh <Rahul.Singh@xxxxxxx>]

Hello Julien,

> On 13 Apr 2022, at 10:44 am, Julien Grall <julien@xxxxxxx> wrote:
> 
> Hi Rahul,
> 
> On 13/04/2022 09:25, Rahul Singh wrote:
>>> On 11 Apr 2022, at 7:01 pm, Julien Grall <julien@xxxxxxx> wrote:
>>>> Signed-off-by: Rahul Singh <rahul.singh@xxxxxxx>
>>>> ---
>>>> docs/designs/dom0less-evtchn.md | 96 +++++++++++++++++++++++++++++++++
>>>> 1 file changed, 96 insertions(+)
>>>> create mode 100644 docs/designs/dom0less-evtchn.md
>>>> diff --git a/docs/designs/dom0less-evtchn.md 
>>>> b/docs/designs/dom0less-evtchn.md
>>>> new file mode 100644
>>>> index 0000000000..6a1b7e8c22
>>>> --- /dev/null
>>>> +++ b/docs/designs/dom0less-evtchn.md
>>>> @@ -0,0 +1,96 @@
>>>> +# Signaling support between two domUs on dom0less system
>>>> +
>>>> +## Current state: Draft version
>>>> +
>>>> +## Proposer(s): Rahul Singh, Bertrand Marquis
>>>> +
>>>> +## Problem Statement:
>>>> +
>>>> +The goal of this work is to define a simple signaling system between Xen 
>>>> guests
>>>> +in dom0less systems.
>>>> +
>>>> +In dom0less system, we cannot make use of xenbus and xenstore that are 
>>>> used in
>>>> +normal systems with dynamic VMs to communicate between domains by 
>>>> providing a
>>>> +bus abstraction for paravirtualized drivers.
>>>> +
>>>> +One possible solution to implement the signaling system between domUs is 
>>>> based
>>>> +on event channels.
>>>> +
>>>> +## Proposal:
>>>> +
>>>> +Event channels are the basic primitive provided by Xen for event 
>>>> notifications.
>>>> +An event channel is a logical connection between 2 domains (more 
>>>> specifically
>>>> +between dom1,port1 and dom2,port2). They essentially store one bit of
>>>> +information, the event of interest is signalled by transitioning this bit 
>>>> from
>>>> +0 to 1. An event is an equivalent of a hardware interrupt.
>>>> +
>>>> +Notifications are received by a guest via an interrupt from Xen to the 
>>>> guest,
>>>> +indicating when an event arrives (setting the bit).
>>> 
>>> I am a bit confused with the description. Are you trying to explain the 
>>> event channel in layman term? If not, then event channel protocol is more 
>>> complicated than that (in particular for fifo).
>> Yes I am trying to explain the event-channel in simple term.
> 
> I would suggest to make that clear and also point to the documentation for 
> the Event Channel ABI (we have a doc for fifo at least).

Ack. 
> 
>>>> +
>>>> +Event channel communication will be established statically between two 
>>>> domU
>>>> +guests before unpausing the domains after domain creation. Event channel
>>>> +connection information between domUs will be passed to XEN via device tree
>>>> +node.
>>> 
>>> Why are we limiting ourself to domUs?
>> As this design is for a dom0less system I mean here to all the domains on 
>> the dom0less system.
>> What I understand is that all domains in the dom0less system are called as 
>> domUs.
> 
> It depends on whether an admin may have specific a dom0 kernel. Looking at 
> Penny series to handle shared memory, it will be possible to create a shared 
> region between dom0 and a dom0less domU. Most likely the user will also want 
> to provide a notification communication.
> 
> So shouldn't we also provide a way to create an event channel between dom0 
> and another domU?

Yes, we can create the event channel between dom0 and dom0less domU. I will add 
this in the next design doc.

Sorry for not explaining it correctly earlier I mean to say that all domains ( 
including the dom0 ) in the dom0less system can
have the static event channel.  

Regards,
Rahul