Re: [XEN PATCH v2 2/5] xen: export get_free_port

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Tue, 25 Jan 2022, Jan Beulich wrote:
> On 25.01.2022 02:10, Stefano Stabellini wrote:
> > On Sun, 23 Jan 2022, Julien Grall wrote:
> >>> diff --git a/xen/common/event_channel.c b/xen/common/event_channel.c
> >>> index da88ad141a..5b0bcaaad4 100644
> >>> --- a/xen/common/event_channel.c
> >>> +++ b/xen/common/event_channel.c
> >>> @@ -232,7 +232,7 @@ int evtchn_allocate_port(struct domain *d, 
> >>> evtchn_port_t
> >>> port)
> >>>       return 0;
> >>>   }
> >>>   -static int get_free_port(struct domain *d)
> >>> +int get_free_port(struct domain *d)
> >>
> >> I dislike the idea to expose get_free_port() (or whichever name we decide)
> >> because this can be easily misused.
> >>
> >> In fact looking at your next patch (#3), you are misusing it as it is 
> >> meant to
> >> be called with d->event_lock. I know this doesn't much matter
> >> in your situation because this is done at boot with no other domains 
> >> running
> >> (or potentially any event channel allocation). However, I still think we
> >> should get the API right.
> >>
> >> I am also not entirely happy of open-coding the allocation in 
> >> domain_build.c.
> >> Instead, I would prefer if we provide a new helper to allocate an unbound
> >> event channel. This would be similar to your v1 (I still need to review the
> >> patch though).
> > 
> > I am happy to go back to v1 and address feedback on that patch. However,
> > I am having difficulties with the implementation. Jan pointed out:
> > 
> > 
> >>> -
> >>> -    chn->state = ECS_UNBOUND;
> >>
> >> This cannot be pulled ahead of the XSM check (or in general anything
> >> potentially resulting in an error), as check_free_port() relies on
> >> ->state remaining ECS_FREE until it is known that the calling function
> >> can't fail anymore.
> > 
> > This makes it difficult to reuse _evtchn_alloc_unbound for the
> > implementation of evtchn_alloc_unbound. In fact, I couldn't find a way
> > to do it.
> > 
> > Instead, I just create a new public function called
> > "evtchn_alloc_unbound" and renamed the existing funtion to
> > "_evtchn_alloc_unbound" (this to addresses Jan's feedback that the
> > static function should be the one starting with "_"). So the function
> > names are inverted compared to v1.
> > 
> > Please let me know if you have any better suggestions.
> > 
> > 
> > diff --git a/xen/common/event_channel.c b/xen/common/event_channel.c
> > index da88ad141a..c6b7dd7fbd 100644
> > --- a/xen/common/event_channel.c
> > +++ b/xen/common/event_channel.c
> > @@ -18,6 +18,7 @@
> >  
> >  #include <xen/init.h>
> >  #include <xen/lib.h>
> > +#include <xen/err.h>
> >  #include <xen/errno.h>
> >  #include <xen/sched.h>
> >  #include <xen/irq.h>
> > @@ -284,7 +285,27 @@ void evtchn_free(struct domain *d, struct evtchn *chn)
> >      xsm_evtchn_close_post(chn);
> >  }
> >  
> > -static int evtchn_alloc_unbound(evtchn_alloc_unbound_t *alloc)
> > +struct evtchn *evtchn_alloc_unbound(struct domain *d, domid_t remote_dom)
> > +{
> > +    struct evtchn *chn;
> > +    int port;
> > +
> > +    if ( (port = get_free_port(d)) < 0 )
> > +        return ERR_PTR(port);
> > +    chn = evtchn_from_port(d, port);
> > +
> > +    evtchn_write_lock(chn);
> > +
> > +    chn->state = ECS_UNBOUND;
> > +    chn->u.unbound.remote_domid = remote_dom;
> > +    evtchn_port_init(d, chn);
> > +
> > +    evtchn_write_unlock(chn);
> > +
> > +    return chn;
> > +}
> > +
> > +static int _evtchn_alloc_unbound(evtchn_alloc_unbound_t *alloc)
> >  {
> >      struct evtchn *chn;
> >      struct domain *d;
> 
> Instead of introducing a clone of this function (with, btw, still
> insufficient locking), did you consider simply using the existing
> evtchn_alloc_unbound() as-is, i.e. with the caller passing
> evtchn_alloc_unbound_t *?

Yes, we tried that first. Unfortunately the (dummy) XSM check cannot
work. This is how we would want to call the function:


    alloc.dom = d->domain_id;
    alloc.remote_dom = hardware_domain->domain_id;
    rc = evtchn_alloc_unbound(&alloc);


This is the implementation of the XSM check:

static XSM_INLINE int xsm_evtchn_unbound(
    XSM_DEFAULT_ARG struct domain *d, struct evtchn *chn, domid_t id2)
{
    XSM_ASSERT_ACTION(XSM_TARGET);
    return xsm_default_action(action, current->domain, d);
}


Note the usage of current->domain. If you have any suggestions on how to
fix it please let me know.