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From: Eric Blake <eblake@redhat.com>
Message-ID: <4de51a49-c0a1-99fb-8a0b-67af3c22e74d@redhat.com>
Date: Tue, 2 Oct 2018 10:21:23 -0500
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Subject: Re: [Qemu-devel] [PATCH 1/2] qapi: support device id for
 x-block-latency-histogram-set
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To: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>, "qemu-devel@nongnu.org" <qemu-devel@nongnu.org>, "qemu-block@nongnu.org" <qemu-block@nongnu.org>
Cc: "armbru@redhat.com" <armbru@redhat.com>, "kwolf@redhat.com" <kwolf@redhat.com>, "mreitz@redhat.com" <mreitz@redhat.com>, Denis Lunev <den@virtuozzo.com>, Nikolay Shirokovskiy <nshirokovskiy@virtuozzo.com>, "jsnow@redhat.com" <jsnow@redhat.com>

On 10/2/18 9:58 AM, Vladimir Sementsov-Ogievskiy wrote:

>>>>> -# @device: device name to set latency histogram for.
>>>>> +# @device: device name to set latency histogram for (better use @i=
d).
>>>>> +#
>>>>> +# @id: The name or QOM path of the guest device.
>>>>

> Hm. It all looks a bit weird. I've just duplicated block_set_io_throttl=
e
> interface:

But that interface was pre-existing under a stable name, so it had to=20
worry about back-compat. Here, we are adding a new command, so we don't=20
have to be stuck with back-compat ugliness.

>=20
> # @device: Block device name (deprecated, use @id instead)
> #
> # @id: The name or QOM path of the guest device (since: 2.8)
>=20
>   =C2=A0=C2=A0=C2=A0 blk =3D qmp_get_blk(arg->has_device ? arg->device =
: NULL,
>   =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=
=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 arg->has_id ? a=
rg->id : NULL,
> errp);
>   =C2=A0=C2=A0=C2=A0 if (!blk) {
> return;
>   =C2=A0=C2=A0=C2=A0 }
>=20
>=20
> So, looks like "The name or" is wrong part. @id can be only path.
>=20
> However, I can call blk_by_name on @id, if blk_by_qdev_id failed..
>=20
> So, variants:
>=20
> 1. both parameters, current code, but fix documentation (will be @id:
> QOM path of the guest device.)
> 2. only @id and only QOM path
> 3. only @id, but fullback to blk_by_name(@id) if failed to find qdev.
>=20
> Any option is ok for me, I don't care. What do you think?

I'm leaning towards 3.

--=20
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org