On 22/01/20 06:41, Markus Armbruster wrote:
> Paolo Bonzini <pbonz...@redhat.com> writes:
>
>> Il mar 21 gen 2020, 15:22 Markus Armbruster <arm...@redhat.com> ha scritto:
>>
>>>> To see it a different way, these are the "C bindings" to QMP, just that
>>>> the implementation is an in-process call rather than RPC. If the QAPI
>>>> code generator was also able to generate Python bindings and the like,
>>>> they would have to be the same for all QEMU binaries, wouldn't they?
>>>
>>> Ommitting the kind of #if we've been discussing is relatively harmless
>>> but what about this one, in qapi-types-block-core.h:
>>>
>>> typedef enum BlockdevDriver {
>>> BLOCKDEV_DRIVER_BLKDEBUG,
>>> [...]
>>> #if defined(CONFIG_REPLICATION)
>>> BLOCKDEV_DRIVER_REPLICATION,
>>> #endif /* defined(CONFIG_REPLICATION) */
>>> [...]
>>> BLOCKDEV_DRIVER__MAX,
>>> } BlockdevDriver;
>>>
>>
>> Well, I don't think this should be conditional at all. Introspection is a
>> tool to detect unsupported features, not working features.
>
> Isn't this what it does? To detect "replication" is unsupported, check
> whether it's absent, and "supported" does not imply "works".
Indeed...
>> KVM will be
>> present in introspection data even if /dev/kvm doesn't exist on your
>> machine or you don't have permission to access it.
>
> Yes.
>
> QAPI/QMP introspection is compile-time static by design. It can't tell
> you more than "this QEMU build supports X".
... and I think it would be fine even if it told you less: "this QEMU
will not give a parse error if X appears in QMP syntax". For example,
QEMU could accept "replication" even if CONFIG_REPLICATION is not
defined and therefore using it would always fail. This would allow
limiting even more use of conditional compilation.
Paolo
