Re: [Qemu-devel] [RFC] sync NIC's MAC maintained in NICConf as soon as emualted NIC's MAC changed in guest

[Michael S. Tsirkin]

On Wed, Sep 25, 2013 at 01:39:48PM +0200, Markus Armbruster wrote:
> "Michael S. Tsirkin" <address@hidden> writes:
> 
> > On Wed, Sep 25, 2013 at 10:14:49AM +0000, Zhanghaoyu (A) wrote:
> >> >> >> >> Hi, all
> >> >> >> >> 
> >> >> >> >> Do live migration if emulated NIC's MAC has been changed, RARP 
> >> >> >> >> with wrong MAC address will broadcast via
> >> >> >> >> qemu_announce_self in destination, so, long time network
> >> >> >> >> disconnection probably happen.
> >> >> >> >
> >> >> >> >Good catch.
> >> >> >> >
> >> >> >> >> I want to do below works to resolve this problem, 1. change 
> >> >> >> >> NICConf's MAC as soon as emulated NIC's MAC changed in guest
> >> >> >> >
> >> >> >> >This will make it impossible to revert it correctly on reset, won't 
> >> >> >> >it?
> >> >> >> >
> >> >> >> You are right.
> >> >> >> virsh reboot <domain>, or virsh reset <domain>, or reboot VM
> >> >> >> from guest, will revert emulated NIC's MAC to original one
> >> >> >> maintained in NICConf.
> >> >> >> During the reboot/reset flow in qemu, emulated NIC's reset handler 
> >> >> >> will sync the MAC address in NICConf to the MAC address in
> >> >> >> emulated NIC structure, e.g., virtio_net_reset sync the MAC
> >> >> >> address in NICConf to VirtIONet'mac.
> >> >> >> 
> >> >> >> BTW, in native scenario, reboot will revert the changed MAC to
> >> >> >> original one, too.
> >> >> >> 
> >> >> >> >> 2. sync NIC's (more precisely, queue) MAC to corresponding 
> >> >> >> >> NICConf in NIC's migration load handler
> >> >> >> >> 
> >> >> >> >> Any better ideas?
> >> >> >> >> 
> >> >> >> >> Thanks,
> >> >> >> >> Zhang Haoyu
> >> >> >> >
> >> >> >> >I think announce needs to poke at the current MAC instead of
> >> >> >> > the default one in NICConf.
> >> >> >> >We can make it respect link down state while we are at it.
> >> >> >> >
> >> >> >> NICConf structures are incorporated in different emulated NIC's 
> >> >> >> structure, e.g., VirtIONet, E1000State_st, RTL8139State, etc.,
> >> >> >> since so many kinds of emulated NICs, they are described by
> >> >> >> different structures, how to find all NICs' current MAC?
> >> >> >> 
> >> >> >> Maybe we can introduce a pointer member 'current_mac' to NICConf 
> >> >> >> structure, which points to the current MAC, then we can find
> >> >> >> all current MACs from NICConf.current_mac.
> >> >> >
> >> >> >I wouldn't make it a pointer, just a buffer with the mac, copy it 
> >> >> >there.
> >> >> >Maybe call it "softmac" that's what it is really.
> >> >> >
> >> >> >> Can we broadcast the RARP with current MAC in NIC's migration
> >> >> >> load handler respectively?
> >> >> >> 
> >> >> >> Thanks,
> >> >> >> Zhang Haoyu
> >> >> >
> >> >> >It's not so simple, you need to retry several times.
> >> >> >
> >> >> Could you make a statement for 'retry several times' ?
> >> >> Is it the process of retrying several times to sending RARP in
> >> >> qemu_announce_self_once?
> >> >
> >> >yes
> >> >
> >> >> 'broadcast the RARP with current MAC in NIC's migration load handler 
> >> >> respectively' is distributing the job of what qemu_announce_self
> >> >> does to every NIC's migration load handler, e.g., in virtio NIC's
> >> >> migration load handler virtio_net_load, we can create a timer to
> >> >> retry several times to send ARAP with current MAC for this NIC,
> >> >> just as same as qemu_announce_self does.
> >> >
> >> >I don't see a lot of value in this yet.
> >> >
> >> In my opinion, it's not so good to introduce a 'softmac' member to
> >> NICConf, which is not essential function of NICConf.
> >
> > Maybe not essential but 100% of hardware we emulate supports softmacs.
> 
> Yes, but NICConf is about NIC *configuration*, not random common NIC
> state.
> 
> We can capture common NIC state in a separate, properly named data type.
> 
> If we want to bunch it together with common configuration in NICConf
> instead, then better rename NICConf to something that actually reflects
> its changed purpose.  I doubt this would be a good idea.

I agree, it should go into NetClientState, not NICConf.
My main point is it's a common thing, let's not duplicate code.

> >> And, distributing the job of what qemu_announce_self does to every
> >> NIC's migration load handler has no disadvantages over
> >> qemu_announce_self,
> >
> > I see some disadvantages, yes.
> > You are going to add code to all devices instead of
> > doing it in one place, there better be a good reason for this.
> 
> Keeping code common to many (most?) NICs factored out makes sense.
> We've started doing that for block devices, in hw/block/block.c.  So
> far, the only code there is about configuration, thus we work with
> BlockConf.
> 
> [...]