Subject: Re: [virtio] [PATCH v2] initialization: add extra device status handshake
"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... Cheers, Rusty.