On Tue, Jun 26, 2018 at 01:06:23PM -0300, Eduardo Habkost wrote: > On Tue, Jun 26, 2018 at 03:05:33PM +0200, Paolo Bonzini wrote: > > On 26/06/2018 14:29, Eduardo Habkost wrote: > > > On Tue, Jun 26, 2018 at 07:57:18AM +0200, Paolo Bonzini wrote: > > >> On 25/06/2018 21:51, Eduardo Habkost wrote: > > >>> In either case, I'm not arguing (yet) for changing the default > > >>> upstream. I'm just arguing for upstream QEMU to not make any > > >>> promises about the default. > > >> > > >> It would be a guest ABI breakage for TCG guests, so it would only apply > > >> to new machine types. I don't think it's worth the complication. > > > > > > That's exactly the point: I want to stop promising a stable guest > > > ABI when the accelerator is omitted, because I see no benefit in > > > wasting energy on this. > > > > On the other hand I see no benefit in changing a default that people are > > obviously not using (since most people use KVM, not TCG). Distros will > > keep shipping, and people will keep using qemu-kvm even if we change the > > default. > > Not changing the default is different from promising we will keep > ABI compatibility if the accelerator is omitted. I just want to > get rid of the latter.
I guess the key question is what is the risk of causing problems if we switch from tcg to kvm:tcg when accelerator is omitted ? Based on what I've seen the likely troublespot would be people who are using QEMU inside a guest with nested-virt enabled. Some nested-virt impls are buggy and will cause L2 guest hangs, or worse L1 host crashes. Then again we're not causing that brokenness - just revealing what already exists. Regards, Daniel -- |: https://berrange.com -o- https://www.flickr.com/photos/dberrange :| |: https://libvirt.org -o- https://fstop138.berrange.com :| |: https://entangle-photo.org -o- https://www.instagram.com/dberrange :|
