#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 bits are:

- `0` – All emulated IDE and SCSI disks (not including CD drives).
- `1` – All emulated NICs.
- `2` – All IDE disks except for the primary master (not including CD drives). This is overridden by bit 0.
- `3` – All emulated NVMe disks.

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 xen/include/public/hvm/pvdrivers.h.

NOTE: The IO ports implementing the unplug protocol are implemented as part of the Xen Platform PCI Device, so if that device is not present in the system then this protocol will not work.

Unplug protocol for old SUSE PVonHVM

During xen-3.0.4 timeframe an unofficial unplug protocol was added to the xen-platform-pci kernel module. The value 0x1 was written to offset 0x4 in the memory region of the Xen Platform PCI Device. This was done unconditionally. The corresponding code in qemu-xen-traditional did an unplug of all NIC, IDE and SCSI devices. This was used in all SUSE releases up to openSUSE 12.3, SLES11SP3. Starting with openSUSE 13.1 and SLES11SP4/SLE12 the official protocol was used.

Unplug protocol for old Novell VMDP

During Xen-3.0 timeframe an unofficial unplug protocol was used in Novells VMDP. Depending on how VMDP was configured it would control all devices, or either NIC or storage. To control all devices the value 0x1 was written to offset 0x4 in the memory region of the Xen Platform PCI Device. This was supposed to unplug NIC, IDE and SCSI devices. If VMDP was configured to control just NIC devices it would write the value 0x2 to offset 0x8. If VMDP was configured to control just storage devices it would write the value 0x1 to offset 0x8. Starting with VMDP version 1.7 (released 2011) the official protocol was used.