1#Xen HVM emulated device unplug protocol 2 3The protocol covers three basic things: 4 5 * Disconnecting emulated devices. 6 * Getting log messages out of the drivers and into dom0. 7 * Allowing dom0 to block the loading of specific drivers. This is 8 intended as a backwards-compatibility thing: if we discover a bug 9 in some old version of the drivers, then rather than working around 10 it in Xen, we have the option of just making those drivers fall 11 back to emulated mode. 12 13The current protocol works like this (from the point of view of 14drivers): 15 161. When the drivers first come up, they check whether the unplug logic 17 is available by reading a two-byte magic number from IO port `0x10`. 18 These should be `0x49d2`. If the magic number doesn't match, the 19 drivers don't do anything. 20 212. The drivers read a one-byte protocol version from IO port `0x12`. If 22 this is 0, skip to 6. 23 243. The drivers write a two-byte product number to IO port `0x12`. At 25 the moment, the only drivers using this protocol are our 26 closed-source ones, which use product number 1. 27 284. The drivers write a four-byte build number to IO port `0x10`. 29 305. The drivers check the magic number by reading two bytes from `0x10` 31 again. If it's changed from `0x49d2` to `0xd249`, the drivers are 32 blacklisted and should not load. 33 346. The drivers write a two-byte bitmask of devices to unplug to IO 35 port `0x10`. The defined bits are: 36 37 * `0` -- All emulated IDE and SCSI disks (not including CD drives). 38 * `1` -- All emulated NICs. 39 * `2` -- All IDE disks except for the primary master (not including CD 40 drives). This is overridden by bit 0. 41 * `3` -- All emulated NVMe disks. 42 43 The relevant emulated devices then disappear from the relevant 44 buses. For most guest operating systems, you want to do this 45 before device enumeration happens. 46 47Once the drivers have checked the magic number, they can send log 48messages to qemu which will be logged to wherever qemu's logs go 49(`/var/log/xen/qemu-dm.log` on normal Xen, dom0 syslog on XenServer). 50These messages are written to IO port `0x12` a byte at a time, and are 51terminated by newlines. There's a fairly aggressive rate limiter on 52these messages, so they shouldn't be used for anything even vaguely 53high-volume, but they're rather useful for debugging and support. 54 55It is still permitted for a driver to use this logging feature if it 56is blacklisted, but *ONLY* if it has checked the magic number and found 57it to be `0x49d2` or `0xd249`. 58 59This isn't exactly a pretty protocol, but it does solve the problem. 60 61The blacklist is, from qemu's point of view, handled mostly through 62xenstore. A driver version is considered to be blacklisted if 63`/mh/driver-blacklist/{product_name}/{build_number}` exists and is 64readable, where `{build_number}` is the build number from step 4 as a 65decimal number. `{product_name}` is a string corresponding to the 66product number in step 3. 67 68The master registry of product names and numbers is in 69xen/include/public/hvm/pvdrivers.h. 70 71NOTE: The IO ports implementing the unplug protocol are implemented 72as part of the Xen Platform PCI Device, so if that device is not 73present in the system then this protocol will not work. 74 75 76Unplug protocol for old SUSE PVonHVM 77 78During xen-3.0.4 timeframe an unofficial unplug protocol was added to 79the xen-platform-pci kernel module. The value 0x1 was written to offset 800x4 in the memory region of the Xen Platform PCI Device. This was done 81unconditionally. The corresponding code in qemu-xen-traditional did an 82unplug of all NIC, IDE and SCSI devices. This was used in all SUSE 83releases up to openSUSE 12.3, SLES11SP3. Starting with openSUSE 13.1 and 84SLES11SP4/SLE12 the official protocol was used. 85 86Unplug protocol for old Novell VMDP 87 88During Xen-3.0 timeframe an unofficial unplug protocol was used in 89Novells VMDP. Depending on how VMDP was configured it would control all 90devices, or either NIC or storage. To control all devices the value 0x1 91was written to offset 0x4 in the memory region of the Xen Platform PCI 92Device. This was supposed to unplug NIC, IDE and SCSI devices. If VMDP 93was configured to control just NIC devices it would write the value 0x2 94to offset 0x8. If VMDP was configured to control just storage devices it 95would write the value 0x1 to offset 0x8. Starting with VMDP version 1.7 96(released 2011) the official protocol was used. 97 98