[Libvirt] Attach qemu commands to an running xml domain

[Moshe Levi]

> -----Original Message-----
> From: Michal Privoznik <mprivozn at redhat.com>
> Sent: Wednesday, March 31, 2021 5:49 PM
> To: Moshe Levi <moshele at nvidia.com>; Daniel P. Berrangé
> <berrange at redhat.com>
> Cc: Waleed Musa <waleedm at nvidia.com>; libvir-list at redhat.com; Adrian
> Chiris <adrianc at nvidia.com>
> Subject: Re: [Libvirt] Attach qemu commands to an running xml domain
> 
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> 
> 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?
In our case it MTU of the VF nerdevice and we set it by OVS. 
> 
> > 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