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Re: [PATCH v2 1/2] qapi/machine.json: add DEVICE_UNPLUG_ERROR QAPI event

From: Daniel Henrique Barboza
Subject: Re: [PATCH v2 1/2] qapi/machine.json: add DEVICE_UNPLUG_ERROR QAPI event
Date: Wed, 16 Jun 2021 13:41:47 -0300
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.10.1

On 6/11/21 9:12 AM, Markus Armbruster wrote:
Daniel Henrique Barboza <danielhb413@gmail.com> writes:

At this moment we only provide one event to report a hotunplug error,
MEM_UNPLUG_ERROR. As of Linux kernel 5.12 and QEMU 6.0.0, the pseries
machine is now able to report unplug errors for other device types, such
as CPUs.

Instead of creating a (device_type)_UNPLUG_ERROR for each new device,
create a generic DEVICE_UNPLUG_ERROR event that can be used by all
unplug errors in the future.

Signed-off-by: Daniel Henrique Barboza <danielhb413@gmail.com>
  qapi/machine.json | 23 +++++++++++++++++++++++
  1 file changed, 23 insertions(+)

diff --git a/qapi/machine.json b/qapi/machine.json
index 58a9c86b36..f0c7e56be0 100644
--- a/qapi/machine.json
+++ b/qapi/machine.json
@@ -1274,3 +1274,26 @@
  { 'event': 'MEM_UNPLUG_ERROR',
    'data': { 'device': 'str', 'msg': 'str' } }
+# Emitted when a device hot unplug error occurs.
+# @device: device name
+# @msg: Informative message
+# Since: 6.1
+# Example:
+# <- { "event": "DEVICE_UNPLUG_ERROR"
+#      "data": { "device": "dimm1",
+#                "msg": "Memory hotunplug rejected by the guest for device 
+#      },
+#      "timestamp": { "seconds": 1615570772, "microseconds": 202844 } }
+{ 'event': 'DEVICE_UNPLUG_ERROR',
+  'data': { 'device': 'str', 'msg': 'str' } }

Missing: update of device_add's doc comment in qdev.json:

     # Notes: When this command completes, the device may not be removed from 
     #        guest.  Hot removal is an operation that requires guest 
     #        This command merely requests that the guest begin the hot removal
     #        process.  Completion of the device removal process is signaled 
with a
     #        DEVICE_DELETED event. Guest reset will automatically complete 
     #        for all devices.


This sure could use some polish.

If I understand things correctly, we're aiming for the following device
unplug protocol:

One thing to note is that DEVICE_UNPLUG_ERROR isn't guaranteed to be send for
every hotunplug error. The event depends on machine/architecture support to
detect a guest side error.

    Unplug the device with device_del (or possibly equivalent)

    If we know we can't unplug the device, fail immediately.  Also emit

I haven't predicted to use this event in those cases as well, although it
seems reasonable to do so now that you mentioned it.

    If possible, unplug the device synchronously and succeed.  Also emit

    Else, initiate unplug and succeed.

    When unplug finishes, emit either DEVICE_DELETED or

Since there's no 100% guarantee that DEVICE_UNPLUG_ERROR will be emitted for
guest side errors, the wording here would be

"When unplug finishes, emit DEVICE_DELETED. A DEVICE_UNPLUG_ERROR can be
emitted if a guest side error was detected"

    For some machines and devices, unplug may never finish.


Any particular reason for not putting event DEVICE_UNPLUG_ERROR next to
DEVICE_DELETED in qdev.json?

Not really. I looked where MEM_UNPLUG_ERROR was declared and put it right
after it. I can change it to qdev.json near DEVICE_DELETED.



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