Re: [PATCH v2 2/2] qapi: Allow introspecting fix for savevm's cooperation with blockdev

[Markus Armbruster <armbru@redhat.com>]

Eric Blake <eblake@redhat.com> writes:

> On 10/1/19 2:34 PM, Markus Armbruster wrote:
>> Peter Krempa <pkrempa@redhat.com> writes:
>>
>>> savevm was buggy as it considered all monitor owned block device nodes
>>
>> Recommend "monitor-owned block device nodes" or "block device nodes
>> owned by a monitor"
>>
>>> for snapshot. With introduction of -blockdev the common usage made all
>>> nodes including protocol nodes monitor owned and thus considered for
>>> snapshot.
>>
>> What exactly is / was the problem?
>
>
> Old way: using QMP add_device, you create a drive backend with two BDS
> (format and protocol) assigned to it; the drive backend has your given
> name, and both BDS have a generated name (beginning with '#').  The
> two BDS are not monitor-owned, rather, the drive is.
>
> New way: using QMP blockdev_add, you create the two BDS manually with
> names of your choice, then plug that blockdev into an unnamed
> blockbackend (the drive no longer needs a name, because you can get at
> everything through the BDS name).  You _could_ do this in one step
> (the QAPI allows self-recursion where you can define both the format
> and protocol in one step), but it is easier to do in two steps (define
> the protocol BDS first, then define the format BDS using a "string"
> name of the protocol BDS instead of a { "driver":..., args... } object
> of the protocol layer.  But by making two calls,  now both BDS are
> monitor-owned.
>
> At snapshot-time, the code currently looks for all monitor-owned nodes
> when deciding what to snapshot.  In the old way, this finds the named
> drive, picks up its associated top-most node, and snapshots the format
> layer.  In the new way, the drive is unnamed so it is skipped, while
> there are two named BDS, but we don't want a snapshot of the protocol
> layer.

So the problem is certain (common & sane) -blockdev use makes savevm
create additional, unwanted snapshots.

Your explanation should be worked into the commit message along with ...

>>>            This was fixed but clients need to be able to detect whether
>>> this fix is present.
>>
>> Fixed where?  Commit hash, if possible.
>
> Pull request:
> https://lists.gnu.org/archive/html/qemu-devel/2019-09/msg04773.html
> (assuming it doesn't need a respin before landing, 8ec72832)

... a pointer to this fix.

Thanks!

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