Re: [Xen-devel] Re: Domain Save-Restore

[Mark Williamson <Mark.Williamson@xxxxxxxxxxxx>]

> memory dump. If that is the case, then hypervisor would copy the image back
> ad somehow restart the VM. Could you enlighten me about how does the
> hypervisor advises (or triggers) the VM so that its running...

Suspending a domain requires some co-operation from the guest OS - there was 
some other recent discussion on the mailing list about this.  You don't just 
pause the guest and save it but you also send it a control interface message 
telling it to suspend.

When the guest OS is ready to be suspended, it makes a "suspend" hypercall 
into Xen.  The domain is then descheduled and dom0 is allowed to suspend the 
domain, saving the memory and register state.  When the domain is restored 
and starts executing again, it sees the "suspend hypercall" as just having 
returned.  From the guest's point of view, it just did a normal hypercall but 
afterwards it executes various resume functions to connect up its devices, 
sort out time, etc.

> how are the 
> registers saved and restored

Offhand I don't know the exact mechanism but IIRC, there's a dom0 op that can 
optionally retrieve a domain's memory state.

> I know these questions might seem little 
> novice but I am still learning Xen and not sure about the structure of the
> code.

Not at all, this is relatively complex stuff.

> Also, is there any official documentation available for the code 
> structure (apart from the interface manual).

Nothing apart from what's in the docs/ directory.  There's one or two text 
files in there also describing stuff.

> How do you put the VM into pause domain? and whats the advantage of putting
> it in pause? does it freeze all the processes and restricts the CPU cycles
> alloted to it?

Pause just stops it being scheduled.

> If that is the case, is there a way to be able to read this 
> frozen state information using hypervisor.

As above, I think you can just use a dom0 op.

> I would really appreciate any help in this regard.

HTH,
Mark

> Chotu
>
>
>
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