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Re: [Qemu-devel] [PATCH v3 3/3] Change other funcitons referring to feat

From: Eric Blake
Subject: Re: [Qemu-devel] [PATCH v3 3/3] Change other funcitons referring to feature_word_info[]
Date: Wed, 5 Sep 2018 12:41:25 -0500
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.9.1

On 09/05/2018 11:44 AM, Eduardo Habkost wrote:
On Wed, Sep 05, 2018 at 10:32:33AM -0500, Eric Blake wrote:
On 09/05/2018 09:10 AM, Eduardo Habkost wrote:
Question to the QAPI schema maintainers below:

-{ 'struct': 'X86CPUFeatureWordInfo',
-  'data': { 'cpuid-input-eax': 'int',
-            '*cpuid-input-ecx': 'int',
-            'cpuid-register': 'X86CPURegister32',
+{ 'struct': 'X86CPUIDFeatureWordInfo',
+  'data': { 'input-eax': 'int',
+            '*input-ecx': 'int',
+            'register': 'X86CPURegister32',
               'features': 'int' } }

You are renaming the struct (which is not visible over the wire), as well as
renaming members within the struct (which is visible, if this type appears
on the wire).

However, 'grep FeatureWordInfo qapi/qapi-introspect.c' has no hits either
before or after this patch (well, first you have to build with my
currently-pending patch as part of Markus' pull request for this trick to
work, https://lists.gnu.org/archive/html/qemu-devel/2018-08/msg05968.html).
Or, even without my patch, grepping for 'input-eax' has no hits as a member
name in any struct.  Which means that there are no QMP commands currently
exposing this struct over the wire.

There is one: qom-get.

Oh, right, the nasty exception that is not visible to introspection :(

AFAICS, this will break the existing libvirt code: it will make
qemuMonitorJSONParseCPUx86Features() error out because it won't
find the "input-eax" field on all X86CPUFeatureWordInfo elements.

No, this change can't break libvirt, since I just proved there is no QMP
command that libvirt could be using that either supplies an input member or
expects an output member named "input-eax" in the first place.

I'm sure it will break libvirt.  libvirt uses
"qom-get path=<cpu-path> property=feature-words" to get
X86FeatureWordInfo data, and it expects the returned data to
have a "input-eax" field.

Yeah, since this type is used in the qom-get backdoor that evades introspection, it's even more important that you follow the backwards-compatibility rules and not rename or delete any previously-mandatory members, since libvirt CAN'T introspect if such changes have happened.

Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org

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