Re: [net PATCH] net: virtio: cap mtu when XDP programs are running

[Jason Wang <jasowang@redhat.com>]

  case.



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.fastabend@intel.com>
>>> ---
>>>    drivers/net/virtio_net.c |    9 ++++++---
>>>    1 file changed, 6 insertions(+), 3 deletions(-)
>>>
>>> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
>>> index 5deeda6..783e842 100644
>>> --- a/drivers/net/virtio_net.c
>>> +++ b/drivers/net/virtio_net.c
>>> @@ -1699,6 +1699,9 @@ static void virtnet_init_settings(struct net_device *dev)
>>>        .set_settings = virtnet_set_settings,
>>>    };
>>>    +#define MIN_MTU ETH_MIN_MTU
>>> +#define MAX_MTU ETH_MAX_MTU
>>> +
>>>    static int virtnet_xdp_set(struct net_device *dev, struct bpf_prog *prog)
>>>    {
>>>        unsigned long int max_sz = PAGE_SIZE - sizeof(struct padded_vnet_hdr);
>>> @@ -1748,6 +1751,9 @@ static int virtnet_xdp_set(struct net_device *dev,
>>> struct bpf_prog *prog)
>>>                virtnet_set_queues(vi, curr_qp);
>>>                return PTR_ERR(prog);
>>>            }
>>> +        dev->max_mtu = max_sz;
>>> +    } else {
>>> +        dev->max_mtu = ETH_MAX_MTU;
>> Or use ETH_DATA_LEN here consider we only allocate a size of GOOD_PACKET_LEN for
>> each small buffer?
>>
>> Thanks
> 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.
>
> As for capping it at GOOD_PACKET_LEN this has the nice benefit of avoiding any
> underestimates in EWMA predictions because it appears min estimates are capped
> at GOOD_PACKET_LEN via get_mergeable_buf_len().

This seems something misunderstanding here, I meant only use 
GOOD_PACKET_LEN for small buffer (which does not use EWMA).

Thanks

>
> Thanks,
> John
>