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Re: [libvirt] [PATCH 0/6] handle NIC_RX_FILTER_CHANGED events from qemu

On 09/30/2014 02:28 PM, Tony Krowiak wrote:
> On 09/24/2014 05:50 AM, Laine Stump wrote:
>> These patches set up an event handler for qemu's NIC_RX_FILTER_CHANGED
>> event, which is sent whenever a guest makes a change to a network
>> device's unicast/multicast filter, vlan table, or MAC address.
>> The handler checks if it is appropriate to respond to the
>> NIC_RX_FILTER_CHANGED event (based on device type and configuration)
>> and takes appropriate action. Currently it checks if the guest
>> interface has been configured with trustGuestRxFilters='yes' (defaults
>> to 'no' for security reasons), and if the host side device is
>> macvtap. If so, and the MAC address on the guest has changed, the MAC
>> address of the macvtap device is changed to match.
>> The result of this is that networking from the guest will continue to
>> work if the mac address of a macvtap-connected network device is
>> changed from within the guest, as long as trustGuestRxFilters='yes'
>> (previously changing the MAC address in the guest would break
>> networking).
>> I still need to add code to compare the old and new unicast and
>> multicast lists and program the filters in the macvtap to match the
>> guest, and to check for a non-empty vlan table and handle that
>> (currently that means just setting promiscuous mode on the macvtap),
>> but that can come in a followup series.
> I was very interested in this patch set because I developed a set of
> patches to respond to the NIC_RX_FILTER_CHANGED event.  I completed
> the patch set several weeks ago and have been awaiting completion of
> our internal review before submitting them to this mailing list.
> Apparently you beat me to the punch.  I have code that compares
> the old and new multicast lists and synchronizes the macvtap filters
> with the guest's.  I can modify my patches to integrate this function
> into what you have provided with this patch set.  Would that be
> agreeable?

Since I've just started working on exactly that, yes :-)

What I'd started was a function virNetDevGetRxFilter(ifname, filter) in
virnetdev.c which would do for the host-side tap/macvtap device what
qemuMonitorQueryRxFilter() does for the guest's interface - retrieve the
current unicast/multicast/vlan tables & modes (using code from
iproute2's "bridge" command as a guide to write equivalent libnl-based
code) and return them in a fully-populated  virNetDevRxFilter object
(that's why I defined that struct in virnetdev.h even though I've so far
used it only in the qemu driver), which would be called from the event
handler; the event handler would compare the two virNetDevRxFilter
objects (the one from the host-side device and the one from the
guest-side device) and issue the necessary commands to make everything
match (well, actually what I've been told is that in the case of vlans,
if the guest has a non-empty vlan table we currently have to just set
the macvtap to promiscuous mode).

It sounds like you're only interested in the multicast list, so if you
wanted to fill in enough to make it do that, your code could be used as
a model to do the unicast list (which seems to be empty most of the time
anyway; as I usually operate above that layer, I'm truthfully not
exactly sure when it's even used).

In the future, this same infrastructure will be used to monitor MAC
address and filter changes for interfaces connected to host bridges via
normal tap devices, so that the bridge can be taken out of "learning"
mode (we then need to manually update the bridge's MAC tables based on
the info from query-rx-filter); this can apparently yield a significant
performance increase.

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