On Tue, Nov 22, 2016 at 06:56:43PM +0100, Maxime Coquelin wrote: > > > On 11/22/2016 03:41 PM, Michael S. Tsirkin wrote: > > On Tue, Nov 22, 2016 at 09:32:01AM -0500, Aaron Conole wrote: > > > Maxime Coquelin <maxime.coque...@redhat.com> writes: > > > > > > > On 11/22/2016 05:07 AM, Jason Wang wrote: > > > > > > > > > > > > > > > On 2016年11月22日 00:23, Michael S. Tsirkin wrote: > > > > > > On Fri, Nov 18, 2016 at 02:21:54PM -0500, Aaron Conole wrote: > > > > > > > Maxime Coquelin <maxime.coque...@redhat.com> writes: > > > > > > > > > > > > > > > On 11/18/2016 07:15 PM, Aaron Conole wrote: > > > > > > > > > Maxime Coquelin <maxime.coque...@redhat.com> writes: > > > > > > > > > > > > > > > > > > > This series implements Virtio spec update from Aaron Conole > > > > > > > > > > which > > > > > > > > > > defines a way for the host to expose its max MTU to the > > > > > > > > > > guest. > > > > > > > > > > > > > > > > > > > > Changes since RFC v1: > > > > > > > > > > --------------------- > > > > > > > > > > - Rebased on top of v2.8.0-rc0 (2.7.90) > > > > > > > > > > - Write MTU unconditionnaly in netcfg to avoid memory > > > > > > > > > > leak (Paolo) > > > > > > > > > > - Add host_mtu property to be able to disable the feature > > > > > > > > > > from QEMU > > > > > > > > > > > > > > > > > > > > Maxime Coquelin (3): > > > > > > > > > > vhost-user: Add new protocol feature MTU > > > > > > > > > > vhost-net: Add new MTU feature support > > > > > > > > > > virtio-net: Add MTU feature support > > > > > > > > > > > > > > > > > > > > hw/net/vhost_net.c | 11 +++++++++++ > > > > > > > > > > hw/net/virtio-net.c | 14 ++++++++++++++ > > > > > > > > > > hw/virtio/vhost-user.c | 11 +++++++++++ > > > > > > > > > > include/hw/virtio/vhost.h | 1 + > > > > > > > > > > include/hw/virtio/virtio-net.h | 1 + > > > > > > > > > > include/net/vhost_net.h | 2 ++ > > > > > > > > > > 6 files changed, 40 insertions(+) > > > > > > > > > I ran this with a VM, but it seems the offered maximum MTU > > > > > > > > > was of > > > > > > > > > value > > > > > > > > > 0 - is this expected with this version? How can I change the > > > > > > > > > offered > > > > > > > > > value? Sorry, I'm not as familiar with QEMU/libvirt side of > > > > > > > > > the > > > > > > > > > world. > > > > > > > > They way I implemented it, the MTU value is to be provided by > > > > > > > > vhost-user process (e.g. OVS/DPDK). I added a Vhost protocol > > > > > > > > feature for this. The sequence is: > > > > > > > > 1. Qemu send VHOST_USER_GET_PROTOCOL_FEATURES request > > > > > > > > 2. DPDK replies with providing supported features > > > > > > > > 3. If DPDK supports VHOST_USER_PROTOCOL_F_MTU, Qemu send > > > > > > > > VHOST_USER_GET_MTU resuest > > > > > > > > 4. DPDK replies with MTU value > > > > > > > > > > > > > > > > Does that make sense? > > > > > > > In the case of a vhost-user backed port, yes (so for instance, if > > > > > > > I use > > > > > > > ovs+dpdk vhost-user in client or server mode). However, what > > > > > > > about the > > > > > > > non-dpdk case, where I still use a virtio-net driver in kernel > > > > > > > and want > > > > > > > to have it backed with, say, a tap device in the host attached to > > > > > > > virbr0 (or some other bridge). It should still pull the mtu from > > > > > > > that > > > > > > > device and offer it, I think. > > > > > > > > > > > > > > > Another possibility would be that we could directly pass the > > > > > > > > MTU value > > > > > > > > to Qemu. It may be easier to implement, and to handle migration. > > > > > > > > Problem is that if we do this, this is not the vSwitch that > > > > > > > > decides the > > > > > > > > MTU to set. > > > > > > > Might be better to determined the mtu by looking at what actually > > > > > > > provides the back-end for the networking. > > > > > > > > > > > > > > > Regards, > > > > > > > > Maxime > > > > > > Right. So in case it's not vhost-user, I would say it has to > > > > > > be specified from QEMU command line. > > > > > > It's probably easier to do the same everywhere, and just send > > > > > > the MTU from qemu to backend. > > > > > > > > > > Or vice-versa? E.g qemu need to be notified if the MTU of tap or > > > > > macvtap > > > > > were changed? > > > > > > I think qemu should just query the MTU at start time and then > > > provide that as the value to the VM. Why specify with command line > > > option? > > > > Passing it on command line is an easy way to make sure we can migrate > > the VM correctly. Also, if we get it from the backend, this requires > > backend changes. In particular, tun/macvtap will need new ioctls. So > > generally that's more work. > > I agree this is easier for migration. > > > > Seems to me like an easy way to get out of sync. > > > > If we send it to the backend, that has a chance to check > > mtu and disconnect on error. > > For vhost-user backend, we can send it the MTU value with a > vhost-user protocol feature. > > For tun/macvtap, how do you do without adding a new ioctl ?
Have management configure same mtu on the backend and in qemu. > > > > > > The spec says the MTU must not be modified by the device once it has > > > > been set. > > > > > > +1 - we don't have an exchange or negotiation mechanism for this. That > > > would require additional bits and communication, and it seems like it > > > isn't worth it. > > > > > > > I think it would require a device reset if MTU came to change. > > > > > > It's just too much work for not enough gain. Guest can change MTU all > > > it wants. Host should just know what it will limit to the guest from > > > the beginning. Maybe I'm too simple, though. > > > > > > -Aaron > > > > I agree it's a good start. > > Thanks, > Maxime
