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Re: [Qemu-devel] [PATCH 2/3] migration: Remove use of old MigrationParam
From: |
Juan Quintela |
Subject: |
Re: [Qemu-devel] [PATCH 2/3] migration: Remove use of old MigrationParams |
Date: |
Tue, 16 May 2017 10:00:20 +0200 |
User-agent: |
Gnus/5.13 (Gnus v5.13) Emacs/25.2 (gnu/linux) |
Markus Armbruster <address@hidden> wrote:
> Juan Quintela <address@hidden> writes:
...
>> As qmp command is asynchronous, you can think that -d is *always* on in
>> QMP O:-)
>
> Yes. The existence of "detach" in QMP is owed to limitations of early
> QMP infrastructure. It's flagged as "invalid" and "should not be
> used" since 2010.
>
> Perhaps we should start a section on QMP in
> <http://wiki.qemu.org/Features/LegacyRemoval>. But I'd like to first
> have a way to communicate "you're using a deprecated feature" warnings
> via QMP.
+1
>> Tristates will complicate it. I still think that:
>>
>> - capability: block_migration
>> - parameter: block_shared
>>
>> Makes more sense, no?
>>
>> If block_migration is not enabled, we ignore the shared parameter. We
>> already do that for other parameters.
>
> My impression as a superficial reader is that migration configuration is
> a historically grown mess. Perhaps we shouldn't try to interpret too
> much intent into it :)
>
> If we redo migration as an instance of the "job" abstraction once we
> have it, then migration configuration & control should become more less
> messy. Of course, the old messes will stay with us for a while in the
> form of backward compatibility messes.
>
> I'm not too particular on how we do the tri-state now, as long as it
> reasonably fits what we have, and is documented clearly.
>>> If the new interface isn't used, the old one still needs to work. If it
>>> is used, the old one either has to do "the right thing", or fail
>>> cleanly.
>>>
>>> We approximate "new interface isn't used" by "block migration is off in
>>> global state". When it is off, the migration command needs to honor its
>>> two flags for compatibility. It must leave block migration off in
>>> global state. Yes, this will complicate the implementation until we
>>> actually remove the deprecated flags. Par for the backward compatility
>>> course.
>>>
>>> When block migration isn't off in global state, we can either
>>>
>>> * let the flags take precedence over the global state (one
>>> interpretation of "do the right thing"), or
>>>
>>> * reject flags that conflict with global state (another interpretation),
>>> or
>>>
>>> * reject *all* flags (fail cleanly).
>>>
>>> The last one looks perfectly servicable to me.
>>
>> Yeap, I think that makes sense. If you use capabilities, parameters,
>> old interface don't work at all.
>>
>> We still have a problem that is what happens if the user does:
>>
>> migrate -b <foo>
>> migrate_cancel (or error)
>> migrate <bar> (without -b)
>>
>> With current patches, it will still use -b. Fixing it requires still
>> anding more code. But I think that this use case is so weird what we
>> should not even care about it.
>
> It's a compatibility break. Whether it's tolerable is a judgement call,
> and not for me to make.
>
> Compatibility breaks need documentation, including release notes.
>
> Say you run migrate with -b by accident (say by recalling a prior
> command from persistent command history, such as qmp-shell's or rlwrap's
> or socat READLINE's), immediately realize what you've done and cancel
> the migration. Are you then stuck with -b forever?
migrate_set_capability block off
and you are done.
But I think that adding documentation would be longer that just adding
the code to clean it.
Later, Juan.
- Re: [Qemu-devel] [PATCH 2/3] migration: Remove use of old MigrationParams, (continued)
- Re: [Qemu-devel] [PATCH 2/3] migration: Remove use of old MigrationParams, Markus Armbruster, 2017/05/15
- Re: [Qemu-devel] [PATCH 2/3] migration: Remove use of old MigrationParams, Juan Quintela, 2017/05/15
- Re: [Qemu-devel] [PATCH 2/3] migration: Remove use of old MigrationParams, Dr. David Alan Gilbert, 2017/05/15
- Re: [Qemu-devel] [PATCH 2/3] migration: Remove use of old MigrationParams, Juan Quintela, 2017/05/15
- Re: [Qemu-devel] [PATCH 2/3] migration: Remove use of old MigrationParams, Dr. David Alan Gilbert, 2017/05/15
- Re: [Qemu-devel] [PATCH 2/3] migration: Remove use of old MigrationParams, Juan Quintela, 2017/05/15
- Re: [Qemu-devel] [PATCH 2/3] migration: Remove use of old MigrationParams, Dr. David Alan Gilbert, 2017/05/15
- Re: [Qemu-devel] [PATCH 2/3] migration: Remove use of old MigrationParams, Juan Quintela, 2017/05/15
- Re: [Qemu-devel] [PATCH 2/3] migration: Remove use of old MigrationParams, Dr. David Alan Gilbert, 2017/05/15
- Re: [Qemu-devel] [PATCH 2/3] migration: Remove use of old MigrationParams, Markus Armbruster, 2017/05/16
- Re: [Qemu-devel] [PATCH 2/3] migration: Remove use of old MigrationParams,
Juan Quintela <=
- Re: [Qemu-devel] [PATCH 2/3] migration: Remove use of old MigrationParams, Peter Xu, 2017/05/15
[Qemu-devel] [PATCH 3/3] migration: Remove old MigrationParams, Juan Quintela, 2017/05/11
Re: [Qemu-devel] [PATCH v2 0/3] Remove old MigrationParams, Hailiang Zhang, 2017/05/11