linux-kernel - Re: [PATCH v6 04/18] xen/pvcalls: xenbus state handling

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Date:   Wed, 5 Jul 2017 13:24:02 -0700 (PDT)
From:   Stefano Stabellini <sstabellini@...nel.org>
To:     Juergen Gross <jgross@...e.com>
cc:     Stefano Stabellini <sstabellini@...nel.org>,
        xen-devel@...ts.xen.org, linux-kernel@...r.kernel.org,
        boris.ostrovsky@...cle.com,
        Stefano Stabellini <stefano@...reto.com>
Subject: Re: [PATCH v6 04/18] xen/pvcalls: xenbus state handling

Many thanks for all the reviews!

On Tue, 4 Jul 2017, Juergen Gross wrote:
> On 03/07/17 23:08, Stefano Stabellini wrote:
> > Introduce the code to handle xenbus state changes.
> > 
> > Implement the probe function for the pvcalls backend. Write the
> > supported versions, max-page-order and function-calls nodes to xenstore,
> > as required by the protocol.
> > 
> > Introduce stub functions for disconnecting/connecting to a frontend.
> > 
> > Signed-off-by: Stefano Stabellini <stefano@...reto.com>
> > Reviewed-by: Boris Ostrovsky <boris.ostrovsky@...cle.com>
> > CC: boris.ostrovsky@...cle.com
> > CC: jgross@...e.com
> > ---
> >  drivers/xen/pvcalls-back.c | 152 +++++++++++++++++++++++++++++++++++++++++++++
> >  1 file changed, 152 insertions(+)
> > 
> > diff --git a/drivers/xen/pvcalls-back.c b/drivers/xen/pvcalls-back.c
> > index 9044cf2..7bce750 100644
> > --- a/drivers/xen/pvcalls-back.c
> > +++ b/drivers/xen/pvcalls-back.c
> > @@ -25,20 +25,172 @@
> >  #include <xen/xenbus.h>
> >  #include <xen/interface/io/pvcalls.h>
> >  
> > +#define PVCALLS_VERSIONS "1"
> > +#define MAX_RING_ORDER XENBUS_MAX_RING_GRANT_ORDER
> > +
> >  struct pvcalls_back_global {
> >  	struct list_head frontends;
> >  	struct semaphore frontends_lock;
> >  } pvcalls_back_global;
> >  
> > +static int backend_connect(struct xenbus_device *dev)
> > +{
> > +	return 0;
> > +}
> > +
> > +static int backend_disconnect(struct xenbus_device *dev)
> > +{
> > +	return 0;
> > +}
> > +
> >  static int pvcalls_back_probe(struct xenbus_device *dev,
> >  			      const struct xenbus_device_id *id)
> >  {
> > +	int err, abort;
> > +	struct xenbus_transaction xbt;
> > +
> > +again:
> > +	abort = 1;
> > +
> > +	err = xenbus_transaction_start(&xbt);
> > +	if (err) {
> > +		pr_warn("%s cannot create xenstore transaction\n", __func__);
> > +		return err;
> > +	}
> > +
> > +	err = xenbus_printf(xbt, dev->nodename, "versions", "%s",
> > +			    PVCALLS_VERSIONS);
> > +	if (err) {
> > +		pr_warn("%s write out 'version' failed\n", __func__);
> 
> s/version/versions/ ?

OK


> > +		goto abort;
> > +	}
> > +
> > +	err = xenbus_printf(xbt, dev->nodename, "max-page-order", "%u",
> > +			    MAX_RING_ORDER);
> > +	if (err) {
> > +		pr_warn("%s write out 'max-page-order' failed\n", __func__);
> > +		goto abort;
> > +	}
> > +
> > +	err = xenbus_printf(xbt, dev->nodename, "function-calls",
> > +			    XENBUS_FUNCTIONS_CALLS);
> > +	if (err) {
> > +		pr_warn("%s write out 'function-calls' failed\n", __func__);
> > +		goto abort;
> > +	}
> > +
> > +	abort = 0;
> > +abort:
> > +	err = xenbus_transaction_end(xbt, abort);
> > +	if (err) {
> > +		if (err == -EAGAIN && !abort)
> 
> Hmm, while I don't think xenbus_transaction_end() will ever
> return -EAGAIN in the abort case I'm not sure you should limit
> the retry loop to the non-abort case.

Realistically, if we want to abort and get -EAGAIN, the best thing to do
is to get out (current behavior). The other option would be to keep
issuing xenbus_transaction_end(xbr, 1) in a loop until it succeeds, but
it seems more fragile to me.


> > +			goto again;
> > +		pr_warn("%s cannot complete xenstore transaction\n", __func__);
> > +		return err;
> > +	}
> > +
> > +	xenbus_switch_state(dev, XenbusStateInitWait);
> 
> I don't think you should switch state in case of abort set, no?

Good point, I'll change that.