Re: [PATCH v3 3/5] xen/arm: configure dom0less domain for enabling xenstore after boot

[Julien Grall <julien@xxxxxxx>]

Hi Stefano,

On 23/03/2022 01:18, Stefano Stabellini wrote:
> On Sat, 29 Jan 2022, Julien Grall wrote:
> > On 28/01/2022 21:33, Stefano Stabellini wrote:
> > > +    rc = evtchn_alloc_unbound(&alloc, true);
> > > +    if ( rc )
> > > +    {
> > > +        printk("Failed allocating event channel for domain\n");
> > > +        return rc;
> > > +    }
> > > +
> > > +    d->arch.hvm.params[HVM_PARAM_STORE_EVTCHN] = alloc.port;
> > > +
> > > +    return 0;
> > > +}
> > > +
> > >    static int __init construct_domU(struct domain *d,
> > >                                     const struct dt_device_node *node)
> > >    {
> > > @@ -3014,7 +3043,19 @@ static int __init construct_domU(struct domain *d,
> > >            return rc;
> > >          if ( kinfo.vpl011 )
> > > +    {
> > >            rc = domain_vpl011_init(d, NULL);
> > > +        if ( rc < 0 )
> > > +            return rc;
> > > +    }
> > > +
> > > +    if ( kinfo.dom0less_enhanced )
> > > +    {
> > > +        rc = alloc_xenstore_evtchn(d);
> > > +        if ( rc < 0 )
> > > +            return rc;
> > > +        d->arch.hvm.params[HVM_PARAM_STORE_PFN] = ~0ULL;
> > I think it would be easy to allocate the page right now. So what prevent us to
> > do it right now?
> Because (as you noted as a comment to the following patch) as soon as
> d->arch.hvm.params[HVM_PARAM_STORE_PFN] is set the guest can continue
> with the initialization and will expect the right data to be set on the
> page.
I think you misunderstood my question. From my understanding, at the 
moment, Linux would break with your proposal. So you need to modify the 
kernel in order to support what you are doing.
IOW, we have room to decide the approach here.

Xenstore protocol has a way to allow (re)connection (see 
docs/mics/xenstore-ring.txt). This feature looks quite suited to what 
you are trying to do here (we want to delay the connection).
The main advantage with this approach is the resources allocation for 
Xenstore will be done in the place and the work in Linux could be 
re-used for non-dom0less domain.
Have you explored it?

> In other words: it is not enough to have the pfn allocated, we
> also need xenstore to initialize it. At that point, it is better to do
> both later from init-dom0less.c.
See above. My main concern with your proposal is the allocation is split 
and this making more difficult to understand the initialization. Could 
you write some documentation how everything is meant to work?
Cheers,

--
Julien Grall