Re: [Xen-devel] [PATCH v4] drivers/tpm: add xen tpmfront interface

[Daniel De Graaf <dgdegra@xxxxxxxxxxxxx>]

On 07/22/2013 11:18 AM, David Vrabel wrote:
> On 01/07/13 22:34, Daniel De Graaf wrote:
> > This is a complete rewrite of the Xen TPM frontend driver, taking
> > advantage of a simplified frontend/backend interface and adding support
> > for cancellation and timeouts.  The backend for this driver is provided
> > by a vTPM stub domain using the interface in Xen 4.3.
> [...]
> > --- /dev/null
> > +++ b/Documentation/xen-tpmfront.txt
> Suggest putting this in Documentation/tpm/.
OK.

> > --- /dev/null
> > +++ b/drivers/char/tpm/xen-tpmfront.c
> [...]
> > +static void backend_changed(struct xenbus_device *dev,
> > +               enum xenbus_state backend_state)
> > +{
> > +       int val;
> Hrm. I don't like how every front/back pair invents their own variation
> of the state machine.
>
> Please document the front and back state machines in
> xen/include/public/io/tpmif.h (and the correspoding copy in Linux).
Is there a standard state machine that would allow devices to avoid
inventing their own?

Otherwise, this is what I plan to add to the header:
/*
 * Xenbus state machine
 *
 * Device open:
 *   1. Both ends start in XenbusStateInitialising
 *   2. Backend transitions to InitWait (frontend does not wait on this step)
 *   3. Frontend populates ring-ref, event-channel, feature-protocol-v2
 *   4. Frontend transitions to Initialised
 *   5. Backend maps grant and event channel, verifies feature-protocol-v2
 *   6. Backend transitions to Connected
 *   7. Frontend verifies feature-protocol-v2, transitions to Connected
 *
 * Device close:
 *   1. State is changed to XenbusStateClosing
 *   2. Frontend transitions to Closed
 *   3. Backend unmaps grant and event, changes state to InitWait
 */

> > +
> > +       switch (backend_state) {
> > +       case XenbusStateInitialised:
> > +       case XenbusStateConnected:
>                 if (dev->state == XenbusStateConnected)
>                         break;
>
> Perhaps?
Sure, although the spurious invocation is not seen with the mini-os
backend and running this code twice is harmless.

> > +               if (xenbus_scanf(XBT_NIL, dev->otherend,
> > +                               "feature-protocol-v2", "%d", &val) < 0)
> > +                       val = 0;
> > +               if (!val) {
> > +                       xenbus_dev_fatal(dev, -EINVAL,
> > +                                       "vTPM protocol 2 required");
> > +                       return;
> > +               }
> > +               xenbus_switch_state(dev, XenbusStateConnected);
> > +               break;
> > +
> > +       case XenbusStateClosing:
> > +       case XenbusStateClosed:
> > +               device_unregister(&dev->dev);
> > +               xenbus_frontend_closed(dev);
> > +               break;
> > +       default:
> > +               break;
> > +       }
> > +}
> David
--
Daniel De Graaf
National Security Agency

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