RE: [PATCH] xen-bus: reduce scope of backend watch

[Paul Durrant <xadimgnik@gmail.com>]

> -----Original Message-----
> From: Anthony PERARD <anthony.perard@citrix.com>
> Sent: 30 September 2020 12:43
> To: Paul Durrant <paul@xen.org>
> Cc: xen-devel@lists.xenproject.org; qemu-devel@nongnu.org; Paul Durrant <pdurrant@amazon.com>; Jerome
> Leseinne <jerome.leseinne@gmail.com>; Edwin Torok <edvin.torok@citrix.com>; Stefano Stabellini
> <sstabellini@kernel.org>
> Subject: Re: [PATCH] xen-bus: reduce scope of backend watch
> 
> On Wed, Sep 23, 2020 at 04:57:31PM +0100, Paul Durrant wrote:
> > From: Paul Durrant <pdurrant@amazon.com>
> >
> > Currently a single watch on /local/domain/X/backend is registered by each
> > QEMU process running in service domain X (where X is usually 0). The purpose
> > of this watch is to ensure that QEMU is notified when the Xen toolstack
> > creates a new device backend area.
> > Such a backend area is specific to a single frontend area created for a
> > specific guest domain and, since each QEMU process is also created to service
> > a specfic guest domain, it is unnecessary and inefficient to notify all QEMU
> > processes.
> > Only the QEMU process associated with the same guest domain need
> > receive the notification. This patch re-factors the watch registration code
> > such that notifications are targetted appropriately.
> >
> > Reported-by: Jerome Leseinne <jerome.leseinne@gmail.com>
> > Signed-off-by: Paul Durrant <pdurrant@amazon.com>
> >
> > diff --git a/hw/xen/xen-backend.c b/hw/xen/xen-backend.c
> > index 10199fb58d..f2711fe4a7 100644
> > --- a/hw/xen/xen-backend.c
> > +++ b/hw/xen/xen-backend.c
> > @@ -41,6 +41,11 @@ static void xen_backend_table_add(XenBackendImpl *impl)
> >      g_hash_table_insert(xen_backend_table_get(), (void *)impl->type, impl);
> >  }
> >
> > +static void **xen_backend_table_keys(unsigned int *count)
> > +{
> > +    return g_hash_table_get_keys_as_array(xen_backend_table_get(), count);
> 
> That could be cast to (const gchar **) as the GLib doc suggest, or (const
> char **) since gchar and char are the same.
> https://developer.gnome.org/glib/stable/glib-Hash-Tables.html#g-hash-table-get-keys-as-array
> 

Ok, I'll re-arrange the const-ing to cast at the inner level.

> > +}
> > +
> >  static const XenBackendImpl *xen_backend_table_lookup(const char *type)
> >  {
> >      return g_hash_table_lookup(xen_backend_table_get(), type);
> > diff --git a/hw/xen/xen-bus.c b/hw/xen/xen-bus.c
> > index 9ce1c9540b..c83da93bf3 100644
> > --- a/hw/xen/xen-bus.c
> > +++ b/hw/xen/xen-bus.c
> > @@ -430,7 +430,13 @@ static void xen_bus_unrealize(BusState *bus)
> >      trace_xen_bus_unrealize();
> >
> >      if (xenbus->backend_watch) {
> > -        xen_bus_remove_watch(xenbus, xenbus->backend_watch, NULL);
> > +        unsigned int i;
> > +
> > +        for (i = 0; i < xenbus->backend_types; i++) {
> > +            xen_bus_remove_watch(xenbus, xenbus->backend_watch[i], NULL);
> 
> We should check if backend_watch[i] is NULL.
> 

Yes, I'll add a check.

> > +        }
> > +
> > +        g_free(xenbus->backend_watch);
> >          xenbus->backend_watch = NULL;
> >      }
> >
> 
> The rest of the patch looks fine. Next improvement is to only look at
> only one backend type in xen_bus_backend_changed() since there is now a
> watch per backend type :-), but that would be for another day.
> 

Might not be too tricky. I'll see if I can come up with a follow-up patch.

  Paul

> Cheers,
> 
> --
> Anthony PERARD