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

[Paolo Bonzini]

On 02/05/2017 12:48, Christian Borntraeger wrote:
>> 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. 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. So my preferred variant
> is

Even for Linux the rules aren't 100% black or white.  There have been
cases where small breakage are introduced, for example 4.12 will break
the BLKDISCARDZEROES ioctl.

We shouldn't treat things are black or white here too.

"-usbdevice" makes sense to deprecate because it brings together a
complicated parsing mechanism, though perhaps we could keep it for
simple devices such as keyboard, mouse, tablet where it's pretty surely
in use in the wild.

"-drive cyls=..." also makes sense to deprecate because it is a very
obscure functionality.

"-drive serial=..." would be nice to deprecate, but I think it's already
less clear that it's not in use in the wild.  Probably it's still on the
side of wanting to eventually remove it.

"-net" is more complicated still.  It's almost surely in use in the
wild, but then probably the users are fairly advanced and (with the
provision that documentation should be updated) I guess we could
eventually get there.

But I really, really see no point in removing --enable-kvm.  It costs
perhaps 10 lines of code that has pretty much no ramifications elsewhere
in the code.  If anything I'd expect "-machine accel" to disappear
before, if ever (in favor of multiple "-accel" options, e.g. "-machine
accel=kvm:tcg" can become "-accel kvm -accel tcg".

Thanks,

Paolo