[libvirt-users] virsh -c lxc:/// setvcpus and <vcpu> configuration fails

[Martin Kletzander]

On Sun, Sep 15, 2019 at 12:21:08PM +0200, info at layer7.net wrote:
>Hi folks!
>
>i created a server with this XML file:
>
><domain type='lxc'>
>  <name>lxctest1</name>
>  <uuid>227bd347-dd1d-4bfd-81e1-01052e91ffe2</uuid>
>  <metadata>
>    <libosinfo:libosinfo
>xmlns:libosinfo="http://libosinfo.org/xmlns/libvirt/domain/1.0">
>      <libosinfo:os id="http://centos.org/centos/6.9"/>
>    </libosinfo:libosinfo>
>  </metadata>
>  <memory unit='KiB'>1024000</memory>
>  <currentMemory unit='KiB'>1024000</currentMemory>
> <vcpu>2</vcpu>
>  <numatune>
>    <memory mode='strict' placement='auto'/>
>  </numatune>
>  <resource>
>    <partition>/machine</partition>
>  </resource>
>  <os>
>    <type arch='x86_64'>exe</type>
>    <init>/sbin/init</init>
>  </os>
>  <idmap>
>    <uid start='0' target='200000' count='65535'/>
>    <gid start='0' target='200000' count='65535'/>
>  </idmap>
>  <features>
>    <privnet/>
>  </features>
>  <clock offset='utc'/>
>  <on_poweroff>destroy</on_poweroff>
>  <on_reboot>restart</on_reboot>
>  <on_crash>destroy</on_crash>
>  <devices>
>    <emulator>/usr/libexec/libvirt_lxc</emulator>
>    <filesystem type='mount' accessmode='mapped'>
>      <source dir='/mnt'/>
>      <target dir='/'/>
>    </filesystem>
>    <interface type='network'>
>      <mac address='00:16:3e:3e:3e:bb'/>
>      <source network='Public Network'/>
>    </interface>
>    <console type='pty'>
>      <target type='lxc' port='0'/>
>    </console>
>  </devices>
></domain>
>
>
>I would expect it to have 2 cpu cores and 1 GB RAM.
>
>
>The RAM config works.
>The CPU config does not:
>

You probably checked /proc/meminfo.  That is provided by libvirt using fuse
filesystem, but at least it is guaranteed thanks to cgroups.  We do not (and I
don't think we even can, at least reliably) do that with cpuinfo.

[...]

>It gives me all CPU's from the host.
>

Although if you ran some perf benchmark it should just cap at 2 cpus.

>I also tried it with
>
><cpu>
>  <topology sockets='1' cores='2' threads='1'/>
></cpu>
>

We should not allow this, IMO.  The reason is that we cannot guarantee or even
emulate this (or even the number of CPUs for that matter).  That's not how
containers work.  We can provide /proc/cpuinfo through a fuse filesystem, but if
the code actually asks the cpu directly there is no layer in which to emulate
the returned information.

>That didnt help too.
>
>I tried to modify the vcpus through virsh:
>
>
>
>#virsh -c lxc:/// setvcpus lxctest1 2
>
>error: this function is not supported by the connection driver:
>virDomainSetVcpus
>

This should not work for LXC, but it does not make sence because if you look at
the XML we allow `<vcpus>2</vcpus>`.

>Which didnt work too.
>
>
>This happens on:
>

Unfortunately anywhere, for the reasons said above.  Ideally it should not be
able to specify <vcpus/>, but rather just <cputune/>, but I don't think we can
change that semantics now that we supported the former for quite some time.
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