[Libvirt] Attach qemu commands to an running xml domain

[Michal Privoznik]

On 3/31/21 4:01 PM, Moshe Levi wrote:
> 
> 
>> -----Original Message-----
>> From: Daniel P. Berrangé <berrange at redhat.com>
>> Sent: Wednesday, March 31, 2021 3:23 PM
>> To: Michal Privoznik <mprivozn at redhat.com>
>> Cc: Waleed Musa <waleedm at nvidia.com>; libvir-list at redhat.com; Moshe
>> Levi <moshele at nvidia.com>; Adrian Chiris <adrianc at nvidia.com>
>> Subject: Re: [Libvirt] Attach qemu commands to an running xml domain

>>
>> I've always thought if we wanted to support use of resources from other
>> namespaces, then that would involve use of a namespace="$PID"
>> attribute to identify the namespace.
>>
>>     <source type='unix' namespace="523532"
>> path='/var/lib/vhost_sockets/sock7f9a971a-cf3' mode='server'/>
>>
>> IOW, libvirt could enter the namespace, and then talk to the OVS db.sock at
>> its normal location.
>>
>> It would be  bit strange having namespace refer to a different NS for the
>> OVS, but using the path=/var/lib/..... from the current namespace, but as
>> long as we define the semantics clearly its not the end of the world.
> Just to better explain the use case here so we have user space vdpa solution which requires the following qemu flags to work:
> 1. page-per-vq=on
> 2. host_mtu
> 
> I understand the adding page-per-vq is feature requested to I am putting that aside.

Correct.

> 
> Regarding the host_mtu we want to use the libvirt xml to set mtu like in [1] so we can set mtu in hot plug.
> Basically we just want that the libvirt xml mtu  will just set the qemu host_mtu.
> Th problem is the this using mtu in libvirt xml will query ovs (it assume that if ovs installed on  the host it needs to set it mtu).
> In our case we have OVS running on  the host for switching and OVS in container to connect the VF netdevice with vhostuser to qemu to just forward traffic from VF to virtio using vdpa)
> 1. One option is to allow to disable the set of mtu in ovs so it will just add the qemu host_mtu flag (we can set the mtu on ovs by openstack)

Don't you need to set the MTU also on the TAP device? Or will it be 
inherited from the OVS bridge?

> 2. Second  option is to change how we query ovs to add --db flag so we can change the query to be on the ovs in the container
> 
> Both option are fine with me.
> 
Michal