docs: update HVM emulated unplug protocol to cover NVMe disks

Recent discussions on xen-devel have highlighted that to properly
support displacing emulated NVMe disks with PV equivalents will need
updates to PV frontends. Therefore it is important that, if an emulated
NVMe disk is exposed to a guest with an existing PV storage frontend,
that frontend does not inadvertently cause unplug of that emulated
disk when unplugging IDE or SCSI disks.

This patch defines a new bit in the mask used to instruct QEMU to unplug
emulated devices which will instruct QEMU to unplug NVMe disks and limits
the semantics of the existing 'all' disk-unplug bit to only IDE and/or SCSI
disks.

Signed-off-by: Paul Durrant <paul.durrant@citrix.com>
Reviewed-by: Wei Liu <wei.liu2@citrix.com>
Acked-by: George Dunlap <george.dunlap@citrix.com>

diff --git a/docs/misc/hvm-emulated-unplug.markdown b/docs/misc/hvm-emulated-unplug.markdown
index 8ead0c5ea4b8d485926569cc62e3886051d00199..f6b27ed04ff72f0094ec6e77b7fb5c8344c79bc0 100644 (file)
--- a/docs/misc/hvm-emulated-unplug.markdown
+++ b/docs/misc/hvm-emulated-unplug.markdown
@@ -32,12 +32,13 @@ drivers):
    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:
+   port `0x10`.  The defined bits are:
 
-  * `1` -- All emulated disks (not including CD drives)
-  * `2` -- All emulated NICs
-  * `4` -- All IDE disks except for the primary master (not including CD
-          drives)
+  * `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