[libvirt] [PATCH] qemu: Add USB and memory balloon by default for aarch64/virt guests

[Daniel P. Berrangé]

On Thu, Jan 25, 2018 at 05:45:51PM +0100, Andrea Bolognani wrote:
> Basically all existing guest types, regardless of the architectur,
> get both a USB controller and a virtio memory balloon by default.
> 
> s390 guests are an exception, for the very good reason that they
> don't support USB at all; the other exception is aarch64/virt
> guests, but in the latter case isn't a compelling reason for them
> to deviate from the widely adopted convention, especially since:
> 
>   * x86/q35 guests, which aarch64/virt guests are for the most
>     part identical to, add these devices by default;
>   * it's trivial to opt out of both default devices by setting
>     model='none';

Except that this requires code changes to downstream applications to
actually do this now, otherwise guests that they were expecting to
not have USB for, now suddenly get it.

>   * higher level applications such as Nova expect at least the
>     USB controller to be present.

This doesn't really help nova in practice, because it needs to operate
correctly with pre-existing libbvirt releases, and even on x86 it should
not be relying on the default USB1 controller, but rather adding a USB2
or USB3 controller.

> So add it by default for newly-defined guests. Existing guests
> will, of course, be left unchanged.

That is still harmful, because an existing mgmt application release that
runs on new libvirt has its guest configs suddenly changed, especially
if using transient guests.

> Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1538637

I consider that bug wontfix.  It is just exchanging one set of problems
for a different set of problems.

Regards,
Daniel
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