Re: Hypervisor to dom0 communication

[Ian Campbell <Ian.Campbell@citrix.com>]

On Tue, 2012-11-13 at 10:26 +0000, Razvan Cojocaru wrote:
> > I think I'd look at the ioreq stuff in preference to the trace stuff.
> > You probably want to use a normal evtchn rather than VIRQ. The code
> > which handles HVM_PARAM_BUFIOREQ_PFN and HVM_PARAM_BUFIOREQ_EVTCHN
> > should give a reasonable starting point (I think, I'm not actually the
> > familiar with this bit of Xen).
> 
> Thanks, I'll look that up.
> 
> > It might also be useful if you describe your actual end-goal -- i.e.
> > what you are ultimately trying to achieve. It may be that there are
> > better approaches or even existing solutions or things which are already
> > close to your needs.
> 
> OK, my immediate end-goal is real-time logging of hypervisor events via 
> a dom0 userspace application. These events are always about a currently 
> running virtual machine, and said virtual machine is paused at the time.
> The userspace tool should be immediately notified, so no polling.

This is very like the ioreq model, where the domain (or maybe just the
vcpu, I'm not sure) is paused while qemu does its thing.

What sort of events are we talking about here? 

> Ideally, in the future, based on information received from the 
> hypervisor, the dom0 application would control other virtual machines 
> (restart them, shut them down, and so on). This, I'm thinking, could 
> either be done by replying to the hypervisor itself via the 
> communication channel and do some work there, or, since the application 
> is in dom0, do this via libxl or some other available userspace tool. 

The right thing to do here is for the userspace tool to communicate with
the toolstack (by whatever means) rather the hypervisor in order to
control the domains.

> However, the paused virtual machine should not be allowed to resume 
> unless the userspace application replies that it is OK to resume.

You might also find some inspiration for this sort of model in the
xenpaging and memshare code.

Ian.