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Subject: Re: [virtio] [PATCH v2] initialization: add extra device status handshake
"Michael S. Tsirkin" <mst@redhat.com> writes: > On Tue, Oct 01, 2013 at 11:19:57AM +0930, Rusty Russell wrote: >> "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... > > 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. That's why the *driver* will need to set DRIVER_OK manually now. I'm thinking Linux would add a BUG_ON(!vq->dev->driver_ok) in virtqueue_kick(). Cheers, Rusty.
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