Xen HVM emulated device unplug protocol

The protocol covers three basic things:

• Disconnecting emulated devices.
• Getting log messages out of the drivers and into dom0.
• Allowing dom0 to block the loading of specific drivers. This is intended as a backwards-compatibility thing: if we discover a bug in some old version of the drivers, then rather than working around it in Xen, we have the option of just making those drivers fall back to emulated mode.

The current protocol works like this (from the point of view of drivers):

1. When the drivers first come up, they check whether the unplug logic is available by reading a two-byte magic number from IO port 0x10. These should be 0x49d2. If the magic number doesn't match, the drivers don't do anything.

2. The drivers read a one-byte protocol version from IO port 0x12. If this is 0, skip to 6.

3. The drivers write a two-byte product number to IO port 0x12. At the moment, the only drivers using this protocol are our closed-source ones, which use product number 1.

4. The drivers write a four-byte build number to IO port 0x10.

5. The drivers check the magic number by reading two bytes from 0x10 again. If it's changed from 0x49d2 to 0xd249, the drivers are blacklisted and should not load.

6. The drivers write a two-byte bitmask of devices to unplug to IO port 0x10. The defined fields are:

   • 1 -- All IDE disks (not including CD drives)
   • 2 -- All emulated NICs
   • 4 -- All IDE disks except for the primary master (not including CD drives)

   The relevant emulated devices then disappear from the relevant buses. For most guest operating systems, you want to do this before device enumeration happens.

Once the drivers have checked the magic number, they can send log messages to qemu which will be logged to wherever qemu's logs go (/var/log/xen/qemu-dm.log on normal Xen, dom0 syslog on XenServer). These messages are written to IO port 0x12 a byte at a time, and are terminated by newlines. There's a fairly aggressive rate limiter on these messages, so they shouldn't be used for anything even vaguely high-volume, but they're rather useful for debugging and support.

It is still permitted for a driver to use this logging feature if it is blacklisted, but ONLY if it has checked the magic number and found it to be 0x49d2 or 0xd249.

This isn't exactly a pretty protocol, but it does solve the problem.

The blacklist is, from qemu's point of view, handled mostly through xenstore. A driver version is considered to be blacklisted if /mh/driver-blacklist/{product_name}/{build_number} exists and is readable, where {build_number} is the build number from step 4 as a decimal number. {product_name} is a string corresponding to the product number in step 3.

The master registry of product names and numbers is in qemu-xen-unstable's xenstore.c.