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
