Re: [Qemu-devel] [libvirt] libvirt default machine-type guarantees? (was Re: [PATCH RFC] hw/pc: set q35 as the default x86 machine)

[Daniel P . Berrangé]

On Tue, Jun 05, 2018 at 11:03:46AM -0300, Eduardo Habkost wrote:
> On Tue, Jun 05, 2018 at 03:44:39PM +0200, Pavel Hrdina wrote:
> > On Tue, Jun 05, 2018 at 10:35:38AM -0300, Eduardo Habkost wrote:
> > > On Tue, Jun 05, 2018 at 02:12:32PM +0100, Daniel P. Berrangé wrote:
> > > > On Tue, Jun 05, 2018 at 10:06:46AM -0300, Eduardo Habkost wrote:
> > > > > (CCing libvir-list)
> > > > > 
> > > > > On Tue, Jun 05, 2018 at 09:43:00AM +0100, Daniel P. Berrangé wrote:
> > > > > > On Tue, Jun 05, 2018 at 09:27:46AM +0200, Gerd Hoffmann wrote:
> > > > > > >   Hi,
> > > > > > > 
> > > > > > > > >   Add to that shortcuts like -cdrom
> > > > > > > > > stop working,
> > > > > > > > 
> > > > > > > > Maybe is fixable.
> > > > > > > 
> > > > > > > Already fixed for ages.
> > > > > > > 
> > > > > > > > I see marking Q35 as the default machine a first step.
> > > > > > > 
> > > > > > > Maybe the better option is to go the arm route:  Just don't 
> > > > > > > define a
> > > > > > > default, so users have to specify pc or q35.  That will make them 
> > > > > > > notice
> > > > > > > there is a world beside 'pc', and we also avoid breaking things
> > > > > > > silently.
> > > > > > 
> > > > > > If QEMU removes the default, then libvirt will have to hardcode
> > > > > > 'pc' as the default to maintain back compatibility, so I don't
> > > > > > think that ends up as a net win
> > > > > 
> > > > > Is there an actual promise to never change the default
> > > > > machine-type documented in the libvirt API, or is this just fear
> > > > > of breaking existing code?
> > > > 
> > > > The risk of breaking things that currently work. Some of the things
> > > > discussed here that risk breaking users if QEMU changes the default,
> > > > have the same risk if libvirt changes the default.
> > > > 
> > > > eg old OS versions that only work with PC, or more commonly pre-existing
> > > > cloud disk images that were built against PC can't be assumed to just
> > > > work against q35, even if the OS in the image supports it.
> > > > 
> > > > If we want to get q35 broadly used for modern OS, then IMHO the best
> > > > option is to record that metadata in libosinfo, as ew do for other
> > > > virtual hardware preferences.  That doesn't fix the problem of disk
> > > > images that might not transparently boot between pc/q35, but at least
> > > > avoids breaking OS that don't support q35 at all.
> > > 
> > > This leads to a more general question: sometimes the defaults
> > > chosen by libvirt are obsolete or broken, and we might want to
> > > change them.
> > > 
> > > Is there a process for changing defaults in libvirt, or libvirt
> > > is bound by past decisions forever?
> > 
> > If the default was always recorded in the domain XML it is safe to
> > change it because it will not affect already existing domains or
> > migration but if the default is not recorded in the domain XML there
> > needs to be a lot of compatibility code.
> 
> That's the opposite of what Daniel said above, isn't it?  The
> machine-type is always recorded in the domain XML, but it's still
> considered unsafe to change.

Yes, I disagree with that Pavel has written here. The domain XML recording
of settings is critical for preserving guest ABI for migration, etc, so
obviously must be stable. Even if there is *no* domain XML yet, however,
libvirt still aims to avoid changes in defaults that are liable to break
an existing mgmt application creating guests in future.

Regards,
Daniel
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