Re: [Qemu-devel] [PATCH] Deprecate '-enable-kvm' and '-enable-hax' in favour of '-accel'

[Thomas Huth]

On 02.05.2017 14:07, Thomas Huth wrote:
> On 02.05.2017 13:59, Daniel P. Berrange wrote:
>> On Tue, May 02, 2017 at 01:26:17PM +0200, Thomas Huth wrote:
>>> On 02.05.2017 12:48, Christian Borntraeger wrote:
>>>> On 05/02/2017 12:37 PM, Thomas Huth wrote:
>>>>> On 02.05.2017 12:32, Christian Borntraeger wrote:
>>>>>> On 05/02/2017 12:06 PM, Thomas Huth wrote:
>>>>>>> The '-enable-...' option do not make too much sense: They do not
>>>>>>> allow additional parameters, using '-accel xxx' is shorter than
>>>>>>> '-enable-xxx' and we're also inconsistent here, since there is
>>>>>>> no '-enable-xen' option available. So let's try to convince the
>>>>>>> users to use '-accel xxx' instead.
>>>>>>
>>>>>> google has 36000 hits for "--enable-kvm" and 18000 hits for "--accel kvm"
>>>>>> So I assume this will affect a lot of setups for only a very small 
>>>>>> benefit.
>>>>>
>>>>> I'm aware of the fact that likely a lot of users are still using
>>>>> -enable-kvm, and I did not mean that we should remove it soon yet. But
>>>>> IMHO we should start now to inform the users that they should slowly
>>>>> switch to the better option "-accel" instead, so that we could maybe
>>>>> remove this "-enable-xxx" stuff sometime in the distant future (let's
>>>>> say QEMU v4.0?).
>>>>
>>>> I come from the Linux side, where "breaking a working setup" will result in
>>>> an angry Linus.
>>>
>>> IMHO that's a good approach, but I think it should primarily applied for
>>> the interfaces that are designed as "API" to other software layers, i.e.
>>> things like QMP and the "-machine" parameter.
>>> "-enable-kvm" is in my eyes rather a "syntactic sugar" convenience
>>> option, so I'd not apply this rule to this option.
>>>
>>>> We certainly have not such strict rules here and we could
>>>> base the decision on the question "how expensive is the maintenance
>>>> of this option?". I think marking it as "legacy option" is fine, but I 
>>>> doubt
>>>> that removing it will make qemu maintenance cheaper.
>>>
>>> Likely not. Actually, I have another point of view in mind here: You
>>> have to consider that QEMU has a *lot* of options, and I think this is
>>> very confusing for the users, especially the new ones. If we always
>>> provide two or three ways to achieve a goal, especially in an
>>> inconsistent way like we do it here, we likely rather create frustration
>>> than joy for the normal users. Providing a clean, straightforward CLI
>>> interface one day could help to improve the user experience quite a bit.
>>
>> The issue is that we have mutually exclusive requirements here. For a
>> straightforward, easy to understand CLI, things like "--enable-kvm" are
>> much quicker to discover & understand than "-machine accel=kvm". The
>> latter gives much more flexibility since it can set all the other opts,
>> but most of those are rarely used by people who are invoking QEMU
>> manually/directly. We need the things like -machine for libvirt and
>> similar, but they are not end user friendly. Killing all the shortcuts
>> like --enable-kvm would cut down the args we expose, but forcing users
>> onto more complex syntax for args like -machine is not improving their
>> lives in general if they don't need that extra flexibility.
> 
> Theoretically yes, but in this case we also have the "-accel kvm" option
> which is IMHO also straighforward and easy to understand, and even
> shorter than "-enable-kvm". If you look at my patch, I did *not* want to
> force the normal users to use "-machine accel=kvm" here, so I don't see
> the point of your argument here.

Apart from that, we never use "-enable-xxx" for any other convenience
option, e.g. we do not use "-enable-usb", but just "-usb". So for a real
convenience option to enable KVM, the option should have been simply
named "-kvm" instead. That "-enable-xxx" stuff is just bad and IMHO does
not really fit into the QEMU world.

 Thomas