diff options
Diffstat (limited to 'virtio-v1.0-wd01-part1-specification.txt')
| -rw-r--r-- | virtio-v1.0-wd01-part1-specification.txt | 57 |
1 files changed, 55 insertions, 2 deletions
diff --git a/virtio-v1.0-wd01-part1-specification.txt b/virtio-v1.0-wd01-part1-specification.txt index 0531125..9b0bbca 100644 --- a/virtio-v1.0-wd01-part1-specification.txt +++ b/virtio-v1.0-wd01-part1-specification.txt @@ -135,9 +135,11 @@ Feature bits are allocated as follows: 0 to 23: Feature bits for the specific device type - 24 to 31: Feature bits reserved for extensions to the queue and + 24 to 32: Feature bits reserved for extensions to the queue and feature negotiation mechanisms + 33 and above: Feature bits resered for future extensions. + For example, feature bit 0 for a network device (i.e. Subsystem Device ID 1) indicates that the device supports checksumming of packets. @@ -146,6 +148,54 @@ In particular, new fields in the device configuration space are indicated by offering a feature bit, so the guest can check before accessing that part of the configuration space. +2.1.2.1 Legacy Interface: A Note on transitions from earlier drafts +-------------------------------------- + +Earlier drafts of this specification (up to 0.9.X) defined a similar, but +different interface between the hypervisor and the guest. +Since these are widely deployed, this specification +accomodates optional features to simplify transition +from these earlier draft interfaces. Specifically: + +Legacy Interface + is an interface specified by an earlier draft of this specification + (up to 0.9.X) +Legacy Device + is a device implemented before this specification was released, + and implementing a legacy interface on the host side +Legacy Driver + is a driver implemented before this specification was released, + and implementing a legacy interface on the guest side + +Legacy devices and legacy drivers are not compliant with this +specification. + +To simplify transition from these earlier draft interfaces, +it is possible to implement: + +Transitional Device + a device supporting both drivers conforming to this + specification, and allowing legacy drivers. + +Transitional Driver + a driver supporting both devices conforming to this + specification, and legacy devices. + +Transitional devices and transitional drivers can be compliant with +this specification (ie. when not operating in legacy mode). + +Devices or drivers with no legacy compatibility are referred to as +non-transitional devices and drivers, respectively. + +Transitional Drivers can detect Legacy Devices by detecting that +the feature bit VIRTIO_F_VERSION_1 is not offered. +Transitional devices can detect Legacy drivers by detecting that +VIRTIO_F_VERSION_1 has not been acknowledged by the driver. + +To make them easier to locate, specification sections documenting +these transitional features are explicitly marked with 'Legacy +Interface' in the section title. + 2.1.3. Configuration Space ------------------------- @@ -2518,7 +2568,7 @@ contents of the event field. The following events are defined: 2.5. Reserved Feature Bits ========================= -Currently there are four device-independent feature bits defined: +Currently there are five device-independent feature bits defined: VIRTIO_F_NOTIFY_ON_EMPTY (24) Negotiating this feature indicates that the driver wants an interrupt if the device runs @@ -2549,6 +2599,9 @@ Currently there are four device-independent feature bits defined: driver should ignore the used_event field; the device should ignore the avail_event field; the flags field is used + VIRTIO_F_VERSION_1(32) This feature must be offered by any device + compliant with this specification, and acknowledged by all device + drivers. In addition, bit 30 is used by qemu's implementation to check for experimental early versions of virtio which did not perform correct feature negotiation, |