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Re: [libvirt] [PATCH] qemu: ensure default machine types don't change if QEMU changes

On Tue, Aug 07, 2018 at 04:57:28PM +0200, Andrea Bolognani wrote:
> On Tue, 2018-08-07 at 15:28 +0100, Daniel P. Berrangé wrote:
> > On Tue, Aug 07, 2018 at 03:54:06PM +0200, Andrea Bolognani wrote:
> > > I wonder if we shouldn't just drop the default machine type handling
> > > altogether at this point, though.
> > 
> > That's impossible as it violates the back compatibility guarantee
> > and will certainly break applications
> In abstract terms, given that the whole point of this exercise is
> shielding our users from changes in QEMU, why wouldn't we go the
> whole way and take QEMU defaults out of the picture entirely?
> I just can't picture a scenario where ignoring the QEMU defaults
> would actually cause issues, since we're basically moving the
> defaults into libvirt with this commit... Can you describe such
> a scenario?

Someone could build a QEMU with the "pc" machine type deleted entirely,
in which case libvirt won't find the default. The best we can do there
is fallback to QEMU's own default.

To avoid that problem we would have to maintain a sorted list of every
single known machine type in every QEMU which gets a bit ridiculous.

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