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. Because A suffers from exactly the same problem if people just blindly switch to a new machine type. > -- > Eduardo
