There may be dierent widths of accesses to the I/O region; the natural access
method for each eld in the virtio header must be used (i.e. 32-bit accesses for
32-bit elds, etc), but the device-specic region can be accessed using any width
accesses, and should obtain the same results.
Note that this is possible because while the virtio header is PCI (i.e. little)
endian, the device-specic region is encoded in the native endian of the guest
(where such distinction is applicable).
2.2.1 Device Initialization Sequence
We start with an overview of device initialization, then expand on the details
of the device and how each step is preformed.
1. Reset the device. This is not required on initial start up.
2. The ACKNOWLEDGE status bit is set: we have noticed the device.
3. The DRIVER status bit is set: we know how to drive the device.
4. Device-specic setup, including reading the Device Feature Bits, discov-
ery of virtqueues for the device, optional MSI-X setup, and reading and
possibly writing the virtio conguration space.
5. The subset of Device Feature Bits understood by the driver is written to
the device.
6. The DRIVER_OK status bit is set.
7. The device can now be used (ie. buers added to the virtqueues)
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If any of these steps go irrecoverably wrong, the guest should set the FAILED
status bit to indicate that it has given up on the device (it can reset the device
later to restart if desired).
We now cover the elds required for general setup in detail.
2.2.2 Virtio Header
The virtio header looks as follows:
Bits 32 32 32 16 16 16 8 8
Read/Write R R+W R+W R R+W R+W R+W R
Purpose
Device Guest Queue Queue Queue Queue Device ISR
Features bits 0:31 Features bits 0:31 Address Size Select Notify Status Status
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Historically, drivers have used the device before steps 5 and 6. This is only allowed if the
driver does not use any features which would alter this early use of the device.
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