Re: [Qemu-devel] [PATCH 1/2 v16] fsdev: add IO throttle support to fsdev devices

From: Eric Blake <eblake@redhat.com>
To: Greg Kurz <groug@kaod.org>
Cc: Stefan Hajnoczi <stefanha@redhat.com>,
	Pradeep Jagadeesh <pradeepkiruvale@gmail.com>,
	"Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com>,
	Alberto Garcia <berto@igalia.com>,
	Pradeep Jagadeesh <pradeep.jagadeesh@huawei.com>,
	qemu-devel@nongnu.org, Fam Zheng <famz@redhat.com>
Subject: Re: [Qemu-devel] [PATCH 1/2 v16] fsdev: add IO throttle support to fsdev devices
Date: Wed, 22 Feb 2017 10:01:09 -0600	[thread overview]
Message-ID: <18735c6a-e21e-6aaf-4283-53f980afae3f@redhat.com> (raw)
In-Reply-To: <20170222144153.131dee9d@bahia.lan>

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On 02/22/2017 07:41 AM, Greg Kurz wrote:
> Eric,
> 
> I fully understand your concern about the missing QMP bits, but given the other
> comments people made on this series, I'd like to move forward and merge it for
> 2.9, without the 'x-' prefixed options. Is it okay with you ?

Yes, I think we'll be okay. Libvirt will just be unable to recognize the
feature until the QMP bits are in place.

>>>> If it cannot be probed via QMP, then libvirt will most likely assume
>>>> that it does not exist.  I guess we're okay having command line only in
>>>> 2.9 if you can't get QMP working, because libvirt will just never drive
>>>> the feature until 2.10 when QMP is available; but then we risk the
>>>> command line subtly changing and breaking someone else that was using
>>>> the command line without QMP.  Maybe the safest approach is to just use
>>>> the 'x-' prefix to the command line portion, until the feature is complete.
>>>>   
>>>
>>> The semantics here are exactly the same as for block devices. The
>>> command line options added to -fsdev are the very same already used
>>> by -drive for years.
>>>
>>> Patch 2/2 in this series even factors them out to a common header file
>>> to be used by fsdev and blockdev. I really don't expect any modification
>>> at all on the command line (nor the other people who reviewed that patch
>>> obviously)... are you suggesting that we should put 2/2 on hold and
>>> use the 'x-' prefix anyway ?  
>>
>> I see these parameter names as stable.  There is little risk that they
>> would change.

We just need to be sure that when we do add QMP, that it doesn't use
names that differ from the command line (blockdev is an example where we
weren't careful in 2.8, and had to rename things in 2.9 to be
consistent, and I want to avoid repeating that mistake).

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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