Re: [PATCH net 1/2] macvlan: forbid L2 fowarding offload for macvtap

From: John Fastabend
Date: Tue Jan 07 2014 - 00:17:29 EST

On 01/06/2014 07:10 PM, Jason Wang wrote:
On 01/06/2014 08:26 PM, Neil Horman wrote:
On Mon, Jan 06, 2014 at 03:54:21PM +0800, Jason Wang wrote:
On 01/06/2014 03:35 PM, John Fastabend wrote:
On 01/05/2014 07:21 PM, Jason Wang wrote:
L2 fowarding offload will bypass the rx handler of real device. This
will make
the packet could not be forwarded to macvtap device. Another problem
is the
dev_hard_start_xmit() called for macvtap does not have any

Fix this by forbidding L2 forwarding for macvtap.

Cc: John Fastabend <john.r.fastabend@xxxxxxxxx>
Cc: Neil Horman <nhorman@xxxxxxxxxxxxx>
Signed-off-by: Jason Wang <jasowang@xxxxxxxxxx>
drivers/net/macvlan.c | 5 ++++-
1 files changed, 4 insertions(+), 1 deletions(-)

I must be missing something.

The lower layer device should set skb->dev to the correct macvtap
device on receive so that in netif_receive_skb_core() the macvtap
handler is hit. Skipping the macvlan receive handler should be OK
because the switching was done by the hardware. If I read macvtap.c
correctly macvlan_common_newlink() is called with 'dev' where 'dev'
is the macvtap device. Any idea what I'm missing? I guess I'll need
to setup a macvtap test case.
Unlike macvlan, macvtap depends on rx handler on the lower device to
work. In this case macvlan_handle_frame() will call macvtap_receive().
So doing netif_receive_skb_core() for macvtap device directly won't work
since we need to forward the packet to userspace instead of kernel.

For net-next.git, it may work since commit
6acf54f1cf0a6747bac9fea26f34cfc5a9029523 let macvtap device register an
rx handler for itself.
I agree, this seems like it should already be fixed with the above commit. With
this the macvlan rx handler should effectively be a no-op as far as the
reception of frames is concerned. As long as the driver sets the dev correctly
to the macvtap device (and it appears to), macvtap will get frames to user
space, regardless of weather the software or hardware did the switching. If
thats the case though, I think the solution is moving that fix to -stable
(pending testing of course), rather than comming up with a new fix.

And what synchronization are you worried about on dev_hard_start_xmit()?
In the L2 forwarding offload case macvlan_open() clears the NETIF_F_LLTX
flag so HARD_TX_LOCK protects the driver txq. We might hit this warning
in dev_queue_xmit() though,

net_crit_ratelimited("Virtual device %s asks to queue packet!\n",

Perhaps we can remove it.
The problem is macvtap does not call dev_queue_xmit() for macvlan
device. It calls macvlan_start_xmit() directly from macvtap_get_user().
So HARD_TX_LOCK was not done for the txq.
This seems to also be fixed by 6acf54f1cf0a6747bac9fea26f34cfc5a9029523.
Macvtap does, as of that commit use dev_queue_xmit for the transmission of
frames to the lowerdevice.

Unfortunately not. This commit has a side effect that it in fact
disables the multiqueue macvtap transmission. Since all macvtap queues
will contend on a single qdisc lock.

They will only contend on a single qdisc lock if the lower device has
1 queue. Perhaps defaulting the L2 forwarding devices to 1queue was a
mistake. But the same issue arises when running macvtap over a
non-multiqueue nic. Or even if you have a multiqueue device and create
many more macvtap queues than the lower device has queues.

Shouldn't the macvtap configuration take into account the lowest level
devices queues? How does using the L2 forwarding device change the
contention issues? Without the L2 forwarding LLTX is enabled but the
qdisc lock, etc is still acquired on the device below the macvlan.

The ixgbe driver as it is currently written can be configured for up to
4 queues by setting numtxqueues when the device is created. I assume
when creating macvtap queues the user needs to account for the number
of queues supported by the lower device.

For L2 forwarding offload itself, more issues need to be addressed for
multiqueue macvtap:

- ndo_dfwd_add_station() can only create queues per device at ndo_open,
but multiqueue macvtap allows user to create and destroy queues at their
will and at any time.

same argument as above, isn't this the same when running macvtap without
the l2 offloads over a real device? I expect you hit the same contention
points when running over a real device.

- it looks that ixgbe has a upper limit of 4 queues per station, but
macvtap currently allows up to 16 queues per device.

The 4 limit was to simplify the code because the queue mapping in the
driver gets complicated if it is greater than 4. We can probably
increase this latter. But sorry reiterating how is this different than
a macvtap on a real device that supports a max of 4 queues?

So more works need to be done and unless those above 3 issues were
addressed, this patch is really needed to make sure macvtap works.

Agreed there is a lot more work here to improve things I'm just not
sure we need to disable this now. Also note its the l2 forwarding
should be disabled by default so a user would have to enable the
feature flag.


John Fastabend Intel Corporation
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