[Xen-devel] [PATCH v2] docs: update HVM emulated unplug protocol to cover NVMe disks

[Paul Durrant <paul.durrant@xxxxxxxxxx>]

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@xxxxxxxxxx>
---
Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
Cc: George Dunlap <George.Dunlap@xxxxxxxxxxxxx>
Cc: Ian Jackson <ian.jackson@xxxxxxxxxxxxx>
Cc: Jan Beulich <jbeulich@xxxxxxxx>
Cc: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
Cc: Stefano Stabellini <sstabellini@xxxxxxxxxx>
Cc: Tim Deegan <tim@xxxxxxx>
Cc: Wei Liu <wei.liu2@xxxxxxxxxx>

v2:
 - Updated commit comment with background rather then referring to
   thread URL.
---
 docs/misc/hvm-emulated-unplug.markdown | 11 ++++++-----
 1 file changed, 6 insertions(+), 5 deletions(-)

diff --git a/docs/misc/hvm-emulated-unplug.markdown 
b/docs/misc/hvm-emulated-unplug.markdown
index 8ead0c5..f6b27ed 100644
--- 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
-- 
2.1.4


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