Re: [Qemu-block] [Qemu-devel] [PATCH 2/2] Add file-posix-dynamic-auto-read-only feature

[Kevin Wolf]

Am 29.03.2019 um 17:19 hat Markus Armbruster geschrieben:
> Markus Armbruster <address@hidden> writes:
> 
> > Markus Armbruster <address@hidden> writes:
> >
> >> Markus Armbruster <address@hidden> writes:
> >>
> >>> Kevin Wolf <address@hidden> writes:
> >>>
> >>>> auto-read-only=on changed its behaviour in file-posix for the 4.0
> >>>> release.
> >>>
> >>> Commit hash, please.
> >>
> >> I guess it's commit 23dece19da4 "file-posix: Make auto-read-only
> >> dynamic".
> >>
> >>>>          This change cannot be detected through the usual mechanisms
> >>>> like schema introspection. Add a new feature to query-qemu-features to
> >>>> allow libvirt to detect the presence of the new behaviour.
> >>>>
> >>>> Signed-off-by: Kevin Wolf <address@hidden>
> > [...]
> >> The "posix" in @file-posix-dynamic-auto-read-only feels weird.  To make
> >> sense of it, you need to know on what kind of host QEMU runs.  If we
> >> ever implement it for Windows hosts, we'd get do add
> >> @file-windows-posix-dynamic-auto-read-only.
> >>
> >> What about instead adding @file-dynamic-auto-read-only with a suitable
> >> compile-time conditional?
> >>
> >> [...]
> >
> > Incremental patch:
> 
> Too fast, use this one.
> 
> diff --git a/qapi/misc.json b/qapi/misc.json
> index df23c54a65..36b5ceb595 100644
> --- a/qapi/misc.json
> +++ b/qapi/misc.json
> @@ -3058,7 +3058,7 @@
>  # Information about support for QEMU features that isn't available through
>  # schema introspection.
>  #
> -# @file-posix-dynamic-auto-read-only:
> +# @file-dynamic-auto-read-only:
>  #   true if auto-read-only=on means that the image file is dynamically 
> reopened
>  #   read-only or read-write depending on whether any writers are attached to
>  #   the node.
> @@ -3066,7 +3066,9 @@
>  # Since: 4.0
>  ##
>  { 'struct': 'QemuFeatures',
> -  'data': { 'file-posix-dynamic-auto-read-only': 'bool' } }
> +  'data': {
> +      'file-dynamic-auto-read-only': { 'type': 'bool',
> +                                       'if': 'defined(CONFIG_POSIX)' } } }

I don't think there is a good reason to make the field conditional. What
should be conditional is its value:

diff --git a/qapi/misc.json b/qapi/misc.json
index df23c54a65..a460fd2acc 100644
--- a/qapi/misc.json
+++ b/qapi/misc.json
@@ -3058,7 +3058,7 @@
 # Information about support for QEMU features that isn't available through
 # schema introspection.
 #
-# @file-posix-dynamic-auto-read-only:
+# @file-dynamic-auto-read-only:
 #   true if auto-read-only=on means that the image file is dynamically reopened
 #   read-only or read-write depending on whether any writers are attached to
 #   the node.
@@ -3066,7 +3066,7 @@
 # Since: 4.0
 ##
 { 'struct': 'QemuFeatures',
-  'data': { 'file-posix-dynamic-auto-read-only': 'bool' } }
+  'data': { 'file-dynamic-auto-read-only': 'bool' } }
 
 ##
 # @query-qemu-features:
diff --git a/qmp.c b/qmp.c
index 2a887c1e7d..3d3c300dc6 100644
--- a/qmp.c
+++ b/qmp.c
@@ -723,7 +723,11 @@ QemuFeatures *qmp_query_qemu_features(Error **errp)
     QemuFeatures *caps = g_new(QemuFeatures, 1);
 
     *caps = (QemuFeatures) {
-        .file_posix_dynamic_auto_read_only = true,
+#ifdef CONFIG_POSIX
+        .file_dynamic_auto_read_only = true,
+#else
+        .file_dynamic_auto_read_only = false,
+#endif
     };
 
     return caps;