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

[Stefan Hajnoczi <stefanha@redhat.com>]

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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 <armbru@redhat.com>
> Cc: Peter Krempa <pkrempa@redhat.com>
> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
> ---
>  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.

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