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

[Daniel P. Berrange]

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.

Regards,
Daniel
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