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

[Julien Grall <julien@xxxxxxx>]

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).
> > > +
> > > +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?
Cheers,

--
Julien Grall