Re: [Xen-devel] [PATCH 2/2] xen-bus: Avoid rewriting identical values to xenstore

[Anthony PERARD <anthony.perard@citrix.com>]

On Wed, Aug 21, 2019 at 04:40:05PM +0100, Paul Durrant wrote:
> > -----Original Message-----
> > From: Anthony PERARD <anthony.perard@citrix.com>
> > Sent: 21 August 2019 10:20
> > To: qemu-devel@nongnu.org
> > Cc: Anthony Perard <anthony.perard@citrix.com>; Stefano Stabellini <sstabellini@kernel.org>; Paul
> > Durrant <Paul.Durrant@citrix.com>; xen-devel@lists.xenproject.org
> > Subject: [PATCH 2/2] xen-bus: Avoid rewriting identical values to xenstore
> > 
> > When QEMU receive a xenstore watch event suggesting that the "state" or
> > "online" status of the frontend or the backend changed, it record this
> > in its own state but it also re-write the value back into xenstore even
> > so there were no changed. This trigger an unnecessary xenstore watch
> > event which QEMU will process again (and maybe the frontend as well).
> > 
> > Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
> > ---
> >  hw/xen/xen-bus.c | 37 ++++++++++++++++++++++++-------------
> >  1 file changed, 24 insertions(+), 13 deletions(-)
> > 
> > diff --git a/hw/xen/xen-bus.c b/hw/xen/xen-bus.c
> > index 982eca4533..c83f07424a 100644
> > --- a/hw/xen/xen-bus.c
> > +++ b/hw/xen/xen-bus.c
> > @@ -481,20 +481,27 @@ static int xen_device_backend_scanf(XenDevice *xendev, const char *key,
> >      return rc;
> >  }
> > 
> > -void xen_device_backend_set_state(XenDevice *xendev,
> > -                                  enum xenbus_state state)
> > +static bool xen_device_backend_record_state(XenDevice *xendev,
> > +                                            enum xenbus_state state)
> >  {
> >      const char *type = object_get_typename(OBJECT(xendev));
> > 
> >      if (xendev->backend_state == state) {
> > -        return;
> > +        return false;
> >      }
> > 
> >      trace_xen_device_backend_state(type, xendev->name,
> >                                     xs_strstate(state));
> > 
> >      xendev->backend_state = state;
> > -    xen_device_backend_printf(xendev, "state", "%u", state);
> > +    return true;
> > +}
> > +
> > +void xen_device_backend_set_state(XenDevice *xendev,
> > +                                  enum xenbus_state state)
> > +{
> > +    if (xen_device_backend_record_state(xendev, state))
> > +        xen_device_backend_printf(xendev, "state", "%u", state);
> >  }
> > 
> >  enum xenbus_state xen_device_backend_get_state(XenDevice *xendev)
> > @@ -502,7 +509,8 @@ enum xenbus_state xen_device_backend_get_state(XenDevice *xendev)
> >      return xendev->backend_state;
> >  }
> > 
> > -static void xen_device_backend_set_online(XenDevice *xendev, bool online)
> > +static void xen_device_backend_set_online(XenDevice *xendev, bool online,
> > +                                          bool export)
> >  {
> >      const char *type = object_get_typename(OBJECT(xendev));
> > 
> > @@ -513,7 +521,8 @@ static void xen_device_backend_set_online(XenDevice *xendev, bool online)
> >      trace_xen_device_backend_online(type, xendev->name, online);
> > 
> >      xendev->backend_online = online;
> > -    xen_device_backend_printf(xendev, "online", "%u", online);
> > +    if (export)
> > +        xen_device_backend_printf(xendev, "online", "%u", online);
> >  }
> >
> 
> Perhaps the behaviour of backend_set_state() and backend_set_online() could be the same? I.e. they both take an 'export' (or perhaps 'publish'?) parameter and only write xenstore if that is true. (I realise that would involve modifying xen-block to pass 'true' as the extra export/publish param, but I think it would be neater overall).

I've actually did it this way for backend_set_state() because the only
reason to update internal states without writing that state into
xenstore is because the xenstore state changed, so
{front,back}end_changed() are the only function that don't want/need to
write the new state into xenstore. I wanted to avoid misuse of the
extra export/publish param in future backend drivers.

As for frontend_set_state() and backend_set_online(), they are only used
in xen-bus.c, creating a new function didn't seems as needed.

I kind of think that maybe I should go further and also have
frontend_record_state() is it could be possible to have frontend drivers
in QEMU. (and maybe record_online so they all looks the same.)

So, would you prefer to have the extra param to *_set_*() that should be
"true" outside of *_changed(), or the extra functions like I did with
backend_{set,record}_state() ?

Thanks,

-- 
Anthony PERARD

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