Re: [Qemu-devel] [PATCH v2 0/5] q35: Remove old machines and unused compat code

[Eduardo Habkost]

On Thu, Feb 04, 2016 at 06:01:50PM +0200, Michael S. Tsirkin wrote:
> On Sat, Jan 23, 2016 at 02:02:08PM -0200, Eduardo Habkost wrote:
> > This is another attempt to remove old q35 machine code. Now I am
> > also removing unused compat code to demonstrate the benefit of
> > throwing away the old code that nobody uses.
> 
> The same thing I said applies - we don't know that nobody uses old q35
> machine types.
> We do know we don't need to migrate to/from them,
> so we can drop compat code.
> But please add aliases so people can still start these machines.

If people use them, they can easily update their configurations.

I will copy and paste the reply Markus sent 4 months ago below.

On Mon, Sep 14, 2015 at 09:18:47AM +0200, Markus Armbruster wrote:
> We've been through this before, but we can go through it once more.
> Choices:
> 
> A. Remove old machine type
> 
>    A guest using it can't be started.  Easy to understand on the host.
>    An error message advising to switch to a newer machine type would be
>    a nice touch.
> 
>    This is a clean break in backward compatibility.  To be mentioned in
>    release notes, obviously.
> 
> B. Change old machine type in a guest-visible way
> 
>    Depending on the nature of the change and the guest, a guest using it
>    either doesn't notice, copes with it successfully, or fails in
>    guest-specific ways.  If the latter, the failure can be "guest
>    hangs", which is much harder to figure out than A.
> 
>    Unless we can *demonstrate* that nothing bad happens for all the
>    guests people actually use with the old machine types, this is a
>    different kind of backward compatibility break.
> 
>    Demonstrating this is feels infeasible to me, but you're welcome to
>    try.
> 
> I could call the difference between the two a tradeoff, but since we've
> been through this before, I'll be more blunt: choosing B robs Peter (the
> guy with guests where badness happens) to pay Paul (the guy with guests
> that cope).  Paul is saved the inconvenience of having to read release
> notes or his logs, and change machine types.  Peter pays for that with
> figuring out WTF his guests are doing now.
> 
> As a user, I'd pick a clean break in backward compatibility over a hack
> that preserves effective compatibility when it works, but breaks it
> uncleanly when it doesn't.
> 
> As a developer, I'm insisting on it.
> 
> So, if you want B, the onus is on *you* to show us why nothing bad will
> happen.
> 

-- 
Eduardo