Re: [Xen-devel] Proposal to allow setting up shared memory areas between VMs from xl config file

[Ian Jackson <ian.jackson@xxxxxxxxxxxxx>]

Stefano Stabellini writes ("Re: Proposal to allow setting up shared memory 
areas between VMs from xl config file"):
> In this scenario, she is going to write to the VM config files of the
> two apps that one page will be shared among the two, so that they can
> send each other messages. She will hard-code the address of the shared
> page in her "bare-metal" app.

Thanks.  This makes some sense.

How do these apps expect to interrupt each other, or do they poll in
the shared memory ?  What I'm getting at with this question is that
perhaps some event channels will need setting up.

> There is no frontend and backend (as in the usual Xen meaning). In some
> configurations one app might be more critical than the other, but in
> some other scenarios they might have the same criticality.

Yes.

> If, as Jan pointed out, we need to call out explicitly which is the
> frontend and which is the backend for page ownership reasons, then I
> suggested we expose that configuration to the user, so that she can
> choose.

Indeed.

ISTM that this scenario doesn't depend on new hypervisor
functionality.  The toolstack could set up the appropriate page
sharing (presumably, this would be done with grants so that the result
is like something the guests could have done themselves.)

I see no objection to the libxl domain configuration file naming
guest-physical addresses for use in this way.

One problem, though, is to do with startup order.  To do this in the
most natural way, one would want to start both guests at once so that
one would know their domids etc.  (Also that avoids questions like
`what if one of them crashes'...)

I'm not sure exactly how to fit this into the libxl model, which
mostly talks about one guest domain at a time; and each guest config
talks about persistent resources, rather than resources which are
somehow exposed by a particular guest.

I think this question is worth exploring to see what shape the right
solution is.

Ian.

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