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

[Eduardo Habkost]

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.

-- 
Eduardo