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

[Michael S. Tsirkin]

On Fri, Feb 05, 2016 at 12:46:11PM -0200, Eduardo Habkost wrote:
> On Fri, Feb 05, 2016 at 12:14:16AM +0200, Michael S. Tsirkin wrote:
> > On Thu, Feb 04, 2016 at 05:09:44PM -0200, Eduardo Habkost wrote:
> > > On Thu, Feb 04, 2016 at 08:02:30PM +0200, Michael S. Tsirkin wrote:
> > > > On Thu, Feb 04, 2016 at 03:16:17PM -0200, Eduardo Habkost wrote:
> > > > > 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.
> > > > > > 
> > > > 
> > > > I agree with the conclusion for option B.  But I think the correct
> > > > solution is not A, it is to analyse changes, maybe even test, and show
> > > > that nothing bad can happen.
> > > 
> > > Do you volunteer for that work?
> > 
> > Nope, sorry. It's your idea, your patchset.
> 
> It's your idea. You are the one proposing to waste resources
> keeping an old machine-type name "working" just because you don't
> want users (who we don't even know if they actually exist) to
> update their configurations on a QEMU upgrade.
> 
> I am proposing the opposite: dropping support to a feature that
> people are unlikely to be using, in a very clear way.

What will happen with installed VMs? Will they break?

> 
> > I am saying, look
> > for some low-hanging fruit.  Find some compat options we can
> > drop without breaking guests, drop just these.  Are there
> > options we need for piix anyway? No point in dropping them at
> > all.
> > 
> > For example, the builtin AML can be dropped since we always use
> > a bios with acpi support now.  It is also trivial to test.
> > 
> > Memory layout is probably ok to change.
> > 
> > Maybe more.
> > 
> > > > 
> > > > Because A suffers from exactly the same problem if people
> > > > just blindly switch to a new machine type.
> > > 
> > > Anything can happen if people change their configurations
> > > blindly.
> > > 
> > > Nobody should change configuration blindly, and that's also
> > > why we shouldn't run a different machine when the user is
> > > asking for an old one. We don't know why the user is asking
> > > for an old machine and we can't make decisions for the user.
> > > Management software might know why an old machine is being
> > > used and might be able to help update the config, but QEMU
> > > doesn't.
> > 
> > What guidance do we provide? Try it and see if it works?  What
> > exactly do we ask user to test? If QEMU developers can't find
> > out whether switching a machine type is safe, what hope is
> > there that management developers can?
> 
> Exactly the same guidance vendors already provide for people that
> want to change machine types today. It depends on who wrote the
> config files and why, and we can't and shouldn't make any
> guesses.

AFAIK that's basically "don't do it".

> -- 
> Eduardo