On 2017年01月10日 13:00, Michael S. Tsirkin wrote:
On Mon, Jan 09, 2017 at 08:25:43PM -0800, John Fastabend wrote:
On 17-01-09 07:55 PM, Michael S. Tsirkin wrote:
On Mon, Jan 09, 2017 at 07:30:34PM -0800, John Fastabend wrote:
On 17-01-09 06:51 PM, Michael S. Tsirkin wrote:
On Tue, Jan 10, 2017 at 10:29:39AM +0800, Jason Wang wrote:
On 2017年01月10日 07:58, Michael S. Tsirkin wrote:
On Mon, Jan 09, 2017 at 03:49:27PM -0800, John Fastabend wrote:
On 17-01-09 03:24 PM, Michael S. Tsirkin wrote:
On Mon, Jan 09, 2017 at 03:13:15PM -0800, John Fastabend wrote:
On 17-01-09 03:05 PM, Michael S. Tsirkin wrote:
On Thu, Jan 05, 2017 at 11:09:14AM +0800, Jason Wang wrote:
On 2017年01月05日 02:57, John Fastabend wrote:
[...]
On 2017年01月04日 00:48, John Fastabend wrote:
On 17-01-02 10:14 PM, Jason Wang wrote:
On 2017年01月03日 06:30, John Fastabend wrote:
XDP programs can not consume multiple pages so we cap the MTU to
avoid this case. Virtio-net however only checks the MTU at XDP
program load and does not block MTU changes after the program
has loaded.
This patch sets/clears the max_mtu value at XDP load/unload time.
Signed-off-by: John Fastabend<john.r.fastab...@intel.com>
---
[...]
OK so this logic is a bit too simply. When it resets the max_mtu I guess it
needs to read the mtu via
virtio_cread16(vdev, ...)
or we may break the negotiated mtu.
Yes, this is a problem (even use ETH_MAX_MTU). We may need a method to notify
the device about the mtu in this case which is not supported by virtio now.
Note this is not really a XDP specific problem. The guest can change the MTU
after init time even without XDP which I assume should ideally result in a
notification if the MTU is negotiated.
Yes, Michael, do you think we need add some mechanism to notify host about
MTU change in this case?
Thanks
Why does host care?
Well the guest will drop packets after mtu has been reduced.
I didn't know. What place in code does this?
hmm in many of the drivers it is convention to use the mtu to set the rx
buffer sizes and a receive side max length filter. For example in the Intel
drivers if a packet with length greater than MTU + some headroom is received we
drop it. I guess in the networking stack RX path though nothing forces this and
virtio doesn't have any code to drop packets on rx size.
In virtio I don't see any existing case currently. In the XDP case though we
need to ensure packets fit in a page for the time being which is why I was
looking at this code and generated this patch.
I'd say just look at the hardware max mtu. Ignore the configured mtu.
Does this work for small buffers consider it always allocate skb with size
of GOOD_PACKET_LEN?
Spec says hardware won't send in packets > max mtu in config space.
I think in any case, we should limit max_mtu to
GOOD_PACKET_LEN for small buffers.
Thanks
XDP seems to have a bunch of weird restrictions, I just
do not like it that the logic spills out to all drivers.
What if someone decides to extend it to two pages in the future?
Recode it all in all drivers ...
Why can't net core enforce mtu?
OK I agree I'll put most the logic in rtnetlink.c when the program is added
or removed.
But, I'm looking at the non-XDP receive_small path now and wondering how does
multiple buffer receives work (e.g. packet larger than GOOD_PACKET_LEN?)
I don't understand the question. Look at add_recvbuf_small,
it adds a tiny buffer for head and then the skb.
Specifically this seems to fail with mergeable buffers disabled
On the host:
# ip link set dev tap0 mtu 9000
# ping 22.2 -s 2048
On the guest:
# insmod ./drivers/net/virtio_net.ko
# ip link set dev eth0 mtu 9000
Why would it work? You are sending a packet larger than ethernet MTU.
Ok, does it mean virtio-net does not support Jumbo frame? And if it
can't work, use MAX_MTU as max_mtu is a bug to me.
With mergeable buffers enabled no problems it works as I expect at least.
We don't expect to get these packets but
mergeable is able to process them anyway.
It's an accident:)
But path MTU discovery indeed benefits from this "accident".
Thanks
