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