On 05/15/2015 06:01 PM, Roger Pau Monné wrote:
> El 12/05/15 a les 13.01, Bob Liu ha escrit:
>> The right place for talk_to_blkback() to query backend features and transport
>> parameters is after backend entered XenbusStateInitWait. There is no problem
>
> talk_to_blkback doesn't gather any backend features, it just publishes
> the features supported by the frontend, which AFAICT can be done at any
1) But talk_tlkback will also allocate and initialize the request ring which
should be done after backend entered XenbusStateInitWait.
Please see the protocol defined in xen/include/public/io/blkif.h:
*****************************************************************************
* Startup *
*****************************************************************************
*
* Tool stack creates front and back nodes with state XenbusStateInitialising.
*
* Front Back
* ================================= =====================================
* XenbusStateInitialising XenbusStateInitialising
* o Query virtual device o Query backend device identification
* properties. data.
* o Setup OS device instance. o Open and validate backend device.
* o Publish backend features and
* transport parameters.
* |
* |
* V
* XenbusStateInitWait
*
* o Query backend features and
* transport parameters.
* o Allocate and initialize the
* request ring.
2) Another problem is after 'mutli-page' ring feature get introduced, we have
to know the max
ring pages supported by backend in setup_blkring().
If backend haven't enter XenbusStateInitWait, we may not query the right value.
E.g.
Frontend Backend
in .probe:
talk_to_blkback()
> setup_blkring()
> xenbus_scanf(max_ring_pages)
in .probe:
xenbus_printf(max_ring_pages)
^^^^ Too late to write the real
value
xenbus_switch_state(dev,
XenbusStateInitWait)
Thank you reviewing these patches!
Regards,
-Bob
> time provided that it's before switching to state XenbusStateInitWait.
> Blkfront doesn't have to wait for the backend to switch to state
> XenbusStateInitWait before publishing the features supported by the
> frontend, which is what talk_to_blkback does.
>
> Roger.
>
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