Re: [PATCH v3 00/13] RFC: [for 5.0]: HMP monitor handlers cleanups

[John Snow <jsnow@redhat.com>]

On 1/28/20 11:47 AM, Dr. David Alan Gilbert wrote:
> * John Snow (jsnow@redhat.com) wrote:
>>
>>
>> On 1/27/20 3:43 PM, Peter Krempa wrote:
>>> On Mon, Jan 27, 2020 at 14:39:02 -0500, John Snow wrote:
>>>>
>>>>
>>>> On 1/27/20 5:36 AM, Maxim Levitsky wrote:
>>>>> This patch series is bunch of cleanups
>>>>> to the hmp monitor code.
>>>>>
>>>>> This series only touched blockdev related hmp handlers.
>>>>>
>>>>> No functional changes expected other that
>>>>> light error message changes by the last patch.
>>>>>
>>>>> This was inspired by this bugzilla:
>>>>> https://bugzilla.redhat.com/show_bug.cgi?id=1719169
>>>>>
>>>>> Basically some users still parse hmp error messages,
>>>>> and they would like to have them prefixed with 'Error:'
>>>>>
>>>>
>>>> HMP isn't meant to be parsed. It's explicitly *not* API or ABI. I do
>>>> like consistency in my UIs (it's useful for human eyes, too), but I'd
>>>> like to know more about the request.
>>>
>>> That's true as long as there's an stable replacement ... see below.
>>>
>>
>> Thanks for the context!
>>
>>>>
>>>> Is this request coming from libvirt? Can we wean them off of this
>>>> interface? What do they need as a replacement?
>>>
>>> There are 5 commands that libvirt still has HMP interfaces for:
>>>
>>> drive_add
>>> drive_del
>>>
>>> savevm
>>> loadvm
>>> delvm
>>>
>>> From upstream point of view there's no value in adding the 'error'
>>> prefix to drive_add/drive_del as libvirt now uses blockdev-add/del QMP
>>> command instead which have implicit error propagation.
>>>
>>
>> As thought.
>>
>>> There are no replacements for the internal snapshot commands, but they
>>> reported the 'error' prefix for some time even before this series.
>>>
>>> Said that, please don't break savevm/loadvm/delvm until a QMP
>>> replacement is added.
>>>
>>
>> Yes, noted. I wonder where userfaultfd write support is these days...
> 
> How would that help you there?
> 

Left at the traffic lights, but there was a thought that we'd be able to
get transactionable save-memory support in QMP if we could use
userfaultfd to do just-in-time copies of memory as needed, until the job
is complete.

This way we could support it properly in QMP and we'd have a replacement
for the HMP version which -- from memory -- is not appropriate for the
QMP channel.

Maybe I imagined this restriction.

--js