On Thu, Nov 1, 2012 at 12:31 PM, Michael S. Tsirkin <m...@redhat.com> wrote: > On Thu, Nov 01, 2012 at 10:53:52AM +0100, Stefan Hajnoczi wrote: >> On Wed, Oct 31, 2012 at 06:34:07PM +0200, Michael S. Tsirkin wrote: >> > On Wed, Oct 31, 2012 at 03:51:08PM +0100, Stefan Hajnoczi wrote: >> > > On Wed, Oct 31, 2012 at 10:57:24AM +0200, Michael S. Tsirkin wrote: >> > > > On Wed, Oct 31, 2012 at 09:07:27AM +0100, Stefan Hajnoczi wrote: >> > > > > On Tue, Oct 30, 2012 at 05:24:06PM +0200, Michael S. Tsirkin wrote: >> > > > > > On Wed, Oct 24, 2012 at 02:49:21PM +0200, Stefan Hajnoczi wrote: >> > > > > > > Allow netdev_del followed by netdev_add to re-peer a NIC and its >> > > > > > > netdev: >> > > > > > > >> > > > > > > (qemu) info network >> > > > > > > virtio-net-pci.0: >> > > > > > > type=nic,model=virtio-net-pci,macaddr=52:54:00:12:34:56 >> > > > > > > \ netdev0: type=user,net=10.0.2.0,restrict=off >> > > > > > > >> > > > > > > (qemu) netdev_del netdev0 >> > > > > > > >> > > > > > > (qemu) netdev_add socket,id=netdev0,listen=:1234 >> > > > > > > >> > > > > > > (qemu) info network >> > > > > > > virtio-net-pci.0: >> > > > > > > type=nic,model=virtio-net-pci,macaddr=52:54:00:12:34:56 >> > > > > > > \ netdev0: type=socket, >> > > > > > > >> > > > > > > This makes it possible to switch netdev while the guest is >> > > > > > > running. It >> > > > > > > is not necessary to reset the NIC. >> > > > > > > >> > > > > > > Note that the NIC's link goes down in netdev_del and back up >> > > > > > > again in >> > > > > > > netdev_add. Therefore the guest becomes aware that the network >> > > > > > > has >> > > > > > > changed, although this depends on the emulated NIC model >> > > > > > > providing link >> > > > > > > status change interrupts. >> > > > > > > >> > > > > > > Signed-off-by: Stefan Hajnoczi <stefa...@redhat.com> >> > > > > > >> > > > > > I'd be surprised if this patch worked when one or both backends >> > > > > > are tap. >> > > > > > tap supports offloads but slirp doesn't, since guest >> > > > > > probes offloads at startup, it assumes it can use offloads. >> > > > > > We also program tap during device operation e.g. on set features. >> > > > > > vhost operation could also be interesting, have not looked into it. >> > > > > >> > > > > Yes, I left a TODO in the RFC patch and described the issue below. >> > > > > We'll have to reject incompatible netdevs. >> > > > >> > > > Ideally, we'd probe all backend capabilities at init time. >> > > > However, looks like we allowed netdev and device creation in any order. >> > > > Can we change this and require netdev always be there before device? >> > > >> > > I don't think the order is a problem. The relaxed order is only >> > > relevant during startup from main() - but in that case we have no >> > > constraints yet anyway. >> > > The problem only occurs when netdev_add is used to create an >> > > incompatible netdev after devices have initialized. We should be able >> > > to check and error out in the code that my RFC patch modifies. If >> > > constraints are violated then netdev_add can fail with an error (the new >> > > netdev is not created and the QMP client needs to try again with a >> > > compatible netdev configuration). >> > > >> > > Maybe I'm misunderstanding your point? >> > > >> > > Stefan >> > >> > OK so if we basically require same type backend then I think it's mostly >> > fine. I was trying to think of a way to allow changing backend type, >> > this becomes messy very quickly. In partuclar macvtap probably >> > shouldn't be swapped with tap even though they are the same type >> > formally. >> >> As long as they are offload-compatible, I think they can be swapped. >> It's up to the user or the management stack to make sure switching >> netdevs makes "sense". So the network may be different and the guest >> needs to DHCP again, but that's the user's problem. > > I think a simple rule like "use same backend type" is better than > an opaque one "are offload-compatible" - user has no idea > which offloads do each of the frontends and backends support. > Also if in future we add offloads to backend X suddenly we > break ability to swap with backend Y. > Let's keep it simple.
Okay, that's a safe constraint that we can start with. If users request more freedom later we can get fancy. Stefan
