Re: [Xen-devel] [PATCH 03/10 v2] xen/arm: vpl011: Enable pl011 emulation for a guest domain in Xen

[Julien Grall <julien.grall@xxxxxxx>]

On 05/05/17 08:10, Bhupinder Thakur wrote:
> Hi Julien,
Hi Bhupinder,

> > > > Hi Jan,
> > > >
> > > > > > @@ -631,6 +632,9 @@ int arch_domain_create(struct domain *d,
> > > > > > unsigned int domcr_flags,
> > > > > >      if ( (rc = domain_vtimer_init(d, config)) != 0 )
> > > > > >          goto fail;
> > > > > >
> > > > > > +    if ( domcr_flags & DOMCRF_vuart )
> > > > > > +        if ( (rc = domain_vpl011_init(d, config)) != 0 )
> > > > > > +            goto fail;
> > > > > >      update_domain_wallclock_time(d);
> > > > I am planning to remove the usage of domain creation flag to check
> > > > whether vuart is enabled/disabled. Please see my next comment. With
> > > > that change, domain_vpl011_init() will be called always. The
> > > > domain_vpl011_init() will check whether vuart is enabled or disabled
> > > > in the config structure passed. If vuart is enabled then it will go
> > > > ahead with vpl011 initialization else it will return without
> > > > initializing vpl011.
> > >
> > > Please don't do that. The arch code decides whether domain_vpl011_init
> > > not the invert.
> >
> > I was wondering whether it would be better to defer the PL011 creation to a
> > domctl. This could be called after the domain is created with all the
> > information required (MMIO region, Console PFN...).
> >
> > This would also make the migration support more trivial as the we will not
> > need to know in advance whether a UART is been used.
> >
> > Any opinions?
> Would there be race condition where the guest tries to access the
> pl011 mmio region (as the domain has been created) but pl011 is not
> initialized yet as domctl is not called? What could be an appropriate
> place to call this domctl? It should be before xenstore is populated
> with vuart ring-ref/port information.
There are no race condition. The domain will only be started when 
everything has been created by calling XEN_DOMCTL_unpausedomain.
The DOMCTL createdomain only initialize the basic structure for the 
domain, after the hypercall the domain is not in state to be run 
because, for instance, the vCPUs were not allocated (see 
XEN_DOMCTL_max_vcpus) and the guest RAM were not populated.
I am not very familiar with the libxl code, but I think 
libxl__arch_domain_create should be a good candidate. I will let Ian and 
Wei confirm that.
Cheers,

--
Julien Grall

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