Re: [Qemu-devel] [PATCH v2 0/5] Removal of deprecated -no-kvm* options

From: Markus Armbruster <armbru@redhat.com>
To: Paolo Bonzini <pbonzini@redhat.com>
Cc: Thomas Huth <thuth@redhat.com>, qemu-devel@nongnu.org
Subject: Re: [Qemu-devel] [PATCH v2 0/5] Removal of deprecated -no-kvm* options
Date: Mon, 07 May 2018 13:56:56 +0200	[thread overview]
Message-ID: <87tvrjvfuf.fsf@dusky.pond.sub.org> (raw)
In-Reply-To: <ff5d2f93-8629-bc78-73a5-c3af38409647@redhat.com> (Paolo Bonzini's message of "Mon, 7 May 2018 13:02:02 +0200")

Paolo Bonzini <pbonzini@redhat.com> writes:

> On 07/05/2018 09:56, Markus Armbruster wrote:
>>>>> I'm not that sure anymore about -no-kvm.  It can come in handy for
>>>>> distros (*cough* RHEL *cough) that only ship a qemu-kvm binary with
>>>>> default accelerator "-machine accel=kvm:tcg".
>>>>>
>>>>> Yeah, "-accel tcg" is only a few characters later, but it is a
>>>>> relatively recent addition.
>>>>
>>>> 2012 is "relatively recent" in the alternate RHEL universe, perhaps :)
>>>
>>> That was the "-machine accel=tcg", but Paolo was rather talking about
>>> "-accel tcg", which has been added last year. Yes, we've got three (!)
>>> ways of dealing with accelerators now: "-M accel=", "-accel" and
>>> "-enable-kvm + -no-kvm". We're very userfriendly, aren't we? ;-)
>>
>> Ugh.  Apparently, vl.c still is a playground without adult supervision.
>
> I'm not sure I get it... "-accel" was added in order to provide
> accelerator options.  If anything, "-machine accel" (added back in 2010)
> is the one that makes little sense these days.

Adding more and more ways to do the same stuff does not improve an
interface.  Interface design needs to be *opinionated*.  If we decide
-machine accel=tcg isn't a nice interface, by all means create a better
one, but as replacement[*], not as addition.

Furthermore:

    tcg: add options for enabling MTTCG

    We know there will be cases where MTTCG won't work until additional work
    is done in the front/back ends to support. It will however be useful to
    be able to turn it on.

    As a result MTTCG will default to off unless the combination is
    supported. However the user can turn it on for the sake of testing.

    Signed-off-by: KONRAD Frederic <fred.konrad@greensocs.com>
    [AJB: move to -accel tcg,thread=multi|single, defaults]
    Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
    Reviewed-by: Richard Henderson <rth@twiddle.net>

I'm sorry, but this us sub-par.  Yes, the commit is also about "enabling
MTTCG", but it also adds a new way to select accelerators, without ever
spelling that out.  It should've been split, and properly described.

[*] Replacements generally involve deprecation and a grace period.