Re: [PATCH v2] Add -build-info and -build-info-json CLI arguments

[Pierrick Bouvier <pierrick.bouvier@linaro.org>]

On 9/24/24 05:08, Daniel P. Berrangé wrote:
> On Tue, Sep 24, 2024 at 01:02:26PM +0100, Alex Bennée wrote:
>> Manos Pitsidianakis <manos.pitsidianakis@linaro.org> writes:
>>
>>> Hello Daniel,
>>>
>>> On Tue, 24 Sept 2024 at 11:45, Daniel P. Berrangé <berrange@redhat.com> wrote:
>>>>
>>>> On Mon, Sep 23, 2024 at 10:09:32PM +0300, Manos Pitsidianakis wrote:
>>>>> Hello Daniel,
>>>>>
>>>>> On Mon, 23 Sep 2024 19:45, "Daniel P. Berrangé" <berrange@redhat.com> wrote:
>>>>>> On Mon, Sep 23, 2024 at 09:05:24AM +0300, Manos Pitsidianakis wrote:
>> <snip>
>>>>>> ie, look a query-audiodevs to discover what audio baxckends are
>>>>>> built-in, don't look for CONFIG_XXX settings related to audio.
>>>>>> If there are gaps in information we can query from QMP, we should
>>>>>> aim to close those gaps.
>>>>>>
>>>>>> IOW, I don't think we should expose this build info info in either
>>>>>> human readable or machine readable format.
>>>>>
>>>>> QAPI/QMP is not the perspective of this patch, this is for people who use
>>>>> custom-built (i.e. not from a distro) binaries and want to be able to
>>>>> identify how it was built. Launching a binary to query stuff is
>>>>> unnecessarily complex for this task, and the info is not generally
>>>>> interesting to the API consumers as you said.
>>>>
>>>> Launching QEMU to talk QMP is our defined public API for querying
>>>> anything about the capabilities of QEMU. We're worked hard to get
>>>> away from providing ad-hoc ways to query QEMU from the command
>>>> line and going back to that is not desirable. It may be slightly
>>>> more complicated, but not by very much.
>>>
>>> Again, this is not a "capabilities discovery" API. It lists the
>>> build-time configuration of the binary. Perhaps we can expose it in a
>>> different way so that people don't end up confused?
>>
>> I think the problem is however much we might say it's not a capabilities
>> discovery API it's very existence encourages users to use it as one.
>>
>> What about a script:
>>
>>    qemu-get-build-info </path/to/qemu>
>>
>> which would launch the binary and query it over QMP? Would that work?
> 
> If this is purely a debugging aid, we could make use of ELF notes to
> just stick the config-host.h content into the binary. This has precedent
> in systemd package notes (https://github.com/systemd/package-notes) and
> is more clearly *NOT* an end user CLI option, nor a public API in QMP.
> 
> Querying is then
> 
>      objdump -j .note.qemu-config-h -s /usrbin/qemu-system-x86_64
> 
> 
> With regards,
> Daniel

This is neat!
@Manos: it would be convenient to still have a script as Alex mentioned, 
as remembering in which notes the information is hidden might be annoying.