linux-kernel - Re: [PATCH v2 04/18] xen/pvcalls: xenbus state handling

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Message-ID: <alpine.DEB.2.10.1706011342320.18759@sstabellini-ThinkPad-X260>
Date:   Thu, 1 Jun 2017 13:54:28 -0700 (PDT)
From:   Stefano Stabellini <sstabellini@...nel.org>
To:     Boris Ostrovsky <boris.ostrovsky@...cle.com>
cc:     Stefano Stabellini <sstabellini@...nel.org>,
        xen-devel@...ts.xen.org, linux-kernel@...r.kernel.org,
        jgross@...e.com, Stefano Stabellini <stefano@...reto.com>
Subject: Re: [PATCH v2 04/18] xen/pvcalls: xenbus state handling

On Thu, 25 May 2017, Boris Ostrovsky wrote:
> On 05/19/2017 07:22 PM, 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>
> > CC: boris.ostrovsky@...cle.com
> > CC: jgross@...e.com
> > ---
> >  drivers/xen/pvcalls-back.c | 135 +++++++++++++++++++++++++++++++++++++++++++++
> >  1 file changed, 135 insertions(+)
> >
> > diff --git a/drivers/xen/pvcalls-back.c b/drivers/xen/pvcalls-back.c
> > index 9044cf2..b4da138 100644
> > --- a/drivers/xen/pvcalls-back.c
> > +++ b/drivers/xen/pvcalls-back.c
> > @@ -25,20 +25,155 @@
> >  #include <xen/xenbus.h>
> >  #include <xen/interface/io/pvcalls.h>
> >  
> > +#define PVCALLS_VERSIONS "1"
> 
> Shouldn't this be in a header file that will be shared with frontends?

I don't think it should: different backends could implement a different
pvcalls version. There is an handshake mechanism to agree on a version
of the protocol to use. It is probably not a good idea to expose this
specific version in the generic pvcalls header.


> > +#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;
> > +
> > +	err = xenbus_printf(XBT_NIL, dev->nodename, "versions", "%s",
> > +			    PVCALLS_VERSIONS);
> > +	if (err) {
> > +		pr_warn("%s write out 'version' failed\n", __func__);
> > +		return -EINVAL;
> 
> err?

OK


> > +	}
> > +
> > +	err = xenbus_printf(XBT_NIL, dev->nodename, "max-page-order", "%u",
> > +			    MAX_RING_ORDER);
> > +	if (err) {
> > +		pr_warn("%s write out 'max-page-order' failed\n", __func__);
> > +		return err;
> > +	}
> > +
> > +	/* "1" means socket, connect, release, bind, listen, accept and poll*/
> 
> This comment is obsolete, or at least should refer to
> XENBUS_FUNCTIONS_CALLS (and is missing a space at the end).

OK


> > +	err = xenbus_printf(XBT_NIL, dev->nodename, "function-calls",
> > +			    XENBUS_FUNCTIONS_CALLS);
> > +	if (err) {
> > +		pr_warn("%s write out 'function-calls' failed\n", __func__);
> > +		return err;
> > +	}
> 
> 
> In case of errors we will end up with all previous entries. I think this
> should be done as a transaction which would be aborted in case of an error.

OK


> > +
> > +	err = xenbus_switch_state(dev, XenbusStateInitWait);
> > +	if (err)
> > +		return err;
> 
> 
> Not sure what to do on an error here (wrt xenstore entries).
> xenbus_switch_state() itself uses transactions. Are we allowed to have
> nested transactions?
> 
> OTOH, xenbus_switch_state() never returns an error, at least now. In
> fact, in most cases we ignore return value.

I don't think we can have nested transactions. For simplicity, I'll
ignore errors from xenbus_switch_state.