Re: [Qemu-devel] [PATCH] qmp: add query-qemu-capabilities

[Markus Armbruster]

Stefan Hajnoczi <address@hidden> writes:

> On Fri, Feb 22, 2019 at 03:32:54PM +0000, Stefan Hajnoczi wrote:
>> QMP clients can usually detect the presence of features via schema
>> introspection.  There are rare features that do not involve schema
>> changes and are therefore impossible to detect with schema
>> introspection.
>> 
>> This patch adds the query-qemu-capabilities command.  It returns a list
>> of capabilities that this QEMU supports.
>> 
>> The decision to make this a command rather than something statically
>> defined in the schema is intentional.  It allows QEMU to decide which
>> capabilities are available at runtime, if necessary.
>> 
>> This new interface is necessary so that QMP clients can discover that
>> migrating disk image files is safe with cache.direct=off on Linux.
>> There is no other way to detect whether or not QEMU supports this.
>> 
>> Cc: Markus Armbruster <address@hidden>
>> Cc: Peter Krempa <address@hidden>
>> Signed-off-by: Stefan Hajnoczi <address@hidden>
>> ---
>>  qapi/misc.json | 42 ++++++++++++++++++++++++++++++++++++++++++
>>  qmp.c          | 18 ++++++++++++++++++
>>  2 files changed, 60 insertions(+)
>
> Markus: query-qemu-features is no longer needed by the page cache
> invalidation feature since a related QMP schema change is being made now
> and clients can detect the feature via that change.
>
> I'm inclined to leave this patch series for now.  Later someone can pick
> it up again if there is a feature that cannot be detected via a schema
> change.

Mekse sense.  I figure we'll likely want the command at some point.  And
that's when we should add it.  Hold on to your git branch until then.