[virt-tools-list] [virt-tool-list][PATCH]virt-manager: use host CPU configuration when creating new VM

Cole Robinson crobinso at redhat.com
Wed Nov 20 13:54:23 UTC 2013

On 11/19/2013 10:39 PM, Chen Hanxiao wrote:
>> -----Original Message-----
>> From: Giuseppe Scrivano [mailto:gscrivan at redhat.com]
>> Sent: Tuesday, November 19, 2013 8:52 PM
>> To: Chen Hanxiao
>> Cc: virt-tools-list at redhat.com
>> Subject: Re: [virt-tools-list] [virt-tool-list][PATCH]virt-manager: use
> host CPU
>> configuration when creating new VM
>> Chen Hanxiao <chenhanxiao at cn.fujitsu.com> writes:
>>> From: Chen Hanxiao <chenhanxiao at cn.fujitsu.com>
>>> This patch will use host CPU configuration automatically
>>> when creating new VM.
>> on what version of virt-manager is your patch based?  I tried to apply
>> it to the current git master branch[1] but I get a conflict.
> My virt-manager git is a little old. I'll try to update it to the latest
>> In addition, I am not sure what is the impact of such a change, what are
>> for example the downsides for a migration if we default to a more
>> restrictive CPU configuration.
> libvirt may refuse to start the VM with a poorer CPU.

Though libvirt will never refuse to start with the host CPU model that it
detects in <capabilities> XML.

> And I think user will not be too surprise when they meet this scenario.
> Just clear the old config, and press the existed button 'Copy host CPU
> configuration'
> will give them better performance and more feature.
> We can doc this for users.

If migration fails because a user used 'copy host CPU', I agree that it's not
a big issue and we can use libvirt's CPU APIs and <capabilities> reporting to
predict when this will fail, and give a warning message about it in the
migration dialog.

However, users will most definitely be surprised if virt-manager defaults to
'Copy host CPU' and their guest crashes on startup:


So right now turning it on by default is not an option. When I add the UI bit,
users can make a one time config change to make 'copy host cpu' their default,
until libvirt and qemu straighten this stuff out.

- Cole

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