Re: [virtio] [PATCH v2] initialization: add extra device status handshake

[Rusty Russell <rusty@au1.ibm.com>]

"Michael S. Tsirkin" <mst@redhat.com> writes:
> On Thu, Sep 26, 2013 at 12:05:19PM +0930, Rusty Russell wrote:
>>    may be a significant (or infinite) delay before setting this
>>    bit.
>>  
>> +  FEATURES_OK (8) Indicates that the driver has acknowledged all the
>> +  features it understands, and feature negotiation is complete.
>> +
>>    DRIVER_OK (4) Indicates that the driver is set up and ready to
>>    drive the device.
>>  
>> @@ -444,23 +447,46 @@ how to communicate with the specific device.
>>  
>>  3. The DRIVER status bit is set: we know how to drive the device.
>>  
>> -4. Device-specific setup, including reading the device feature
>> -  bits, discovery of virtqueues for the device, optional per-bus
>> -  setup, and reading and possibly writing the device's virtio
>> -  configuration space.
>> +4. Device feature bits are read, and the the subset of feature bits
>> +   understood by the OS and driver is written to the device.
>> +
>> +5. The FEATURES_OK status bit is set.
>>  
>> -5. The subset of device feature bits understood by the driver is
>> -   written to the device.
>> +6. The status byte is re-read to ensure the FEATURES_OK bit is still
>> +   set: otherwise, the device does not support our subset of features
>> +   and the device is unusable.
>>  
>> -6. The DRIVER_OK status bit is set.
>> +7. Device-specific setup, including discovery of virtqueues for the
>> +   device, optional per-bus setup, reading and possibly writing the
>> +   device's virtio configuration space, and population of virtqueues.
>>  
>> -7. The device can now be used (ie. buffers added to the
>> -   virtqueues)[4]
>> +8. The DRIVER_OK status bit is set.  At this point the device is
>> +   "live".
>
> What exactly does this imply?
> Should device consume buffers before it's live, or should
> it postpone this until DRIVER_OK?

The device MUST NOT consume buffers before DRIVER_OK.
    - The driver is free to rip those descriptors back out if it wants
    to clean up and fail, for example.
    
The driver MUST NOT notify the device before DRIVER_OK.
    - It's not ready yet...

Cheers,
Rusty.