On Tue, Jan 09, 2018 at 04:37:10PM +0100, Jiri Denemark wrote:
> On Tue, Jan 09, 2018 at 15:32:54 +0100, Kashyap Chamarthy wrote:
[...]
> > But doesn't tell *what* the default value is. It is check='partial'.
> > Mention it so.
[...]
> NACK
>
> As I said on IRC, the default differs with guest architecture ('partial'
> for x86_64, 'none' for all others).
Yep, I missed to distinguish that detail.
> Moreover this only applies to QEMU driver. Oh and it even changes
> based on the mode attribute to make not make it easy. But this is
> really an implementation detail. Libvirt will just use a suitable
> default to stay compatible with older releases.
That's all good. If you think nothing needs to be clarified here,
that's OK. (Some admins who operate a lot of VMs with libvirt will
disagree with you, though.)
And as we saw on IRC, `virt-install` muddies the water in this case by
gratuitously adding a CPU parameter with check='full' (even if a user
didn't explicitly specify it) when you simply import an image and look
at the run-time guest XML:
$> virt-install --name vm1 --ram 2048 \
--disk path=./vm1.qcow2,format=qcow2 --nographics \
--import --os-variant fedora27
$> virsh dumpxml vm1 | grep check
<cpu mode='custom' match='exact' check='full'>
(You might, fairly, argue here that: "Well, that's a bug in
`virt-install`, go complain there.")
It all comes across as messy to an unsuspecting admin. (If a competent
admin finds it confusing, good luck to new admins who want to wrap their
head around this.)
- - -
Background for others reading: The admin who reported this was confused
when he was creating guests with `virt-install`, which adds check='full'
(as noted earlier), and the guest throws:
error: Failed to start domain foo.org
error: operation failed: guest CPU doesn't match specification: extra features: vme,arat
So "somehow" QEMU added the CPU features 'vme' and 'arat' by itself, now
you have to specify them in libvirt. So the admin ended up with a `sed`
one-liner that updates the guest XML with the missing features:
sed -i -e "s-</cpu>-<feature policy='require' name='vme'/></cpu>-"
sed -i -e "s-</cpu>-<feature policy='require' name='arat'/></cpu>-"
Versions: libvirt 3.2 and QEMU 2.9
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