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Subject: Re: [virtio] [PATCH v2] initialization: add extra device status handshake
On Tue, Oct 01, 2013 at 11:19:57AM +0930, Rusty Russell wrote: > "Michael S. Tsirkin" <firstname.lastname@example.org> 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) > >> +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... This last one will need checks on data path in driver: if (vq->dev->driver_ok) vq_kick(); consider that once we register a netdev it can start sending packets. But then what kicks vqs after DRIVER_OK? Let's add text that device should poll all vqs when DRIVER_OK is set? Or do you want to do this from driver? > Cheers, > Rusty.