Re: [dpdk-dev] [v4] net/virtio: set offload flag for jumbo frames

[Ilya Maximets <i.maximets@samsung.com>]

On 01.02.2019 18:13, Jens Freimann wrote:
> On Fri, Feb 01, 2019 at 05:27:37PM +0300, Ilya Maximets wrote:
>> On 01.02.2019 13:03, Jens Freimann wrote:
>>> +    if (host_features & (1ULL << VIRTIO_NET_F_MTU)) {
>>> +        uint32_t ether_hdr_len = ETHER_HDR_LEN + VLAN_TAG_LEN +
>>> +            hw->vtnet_hdr_size;
>>> +        if (dev->data->dev_conf.rxmode.max_rx_pkt_len <=
>>> +                hw->max_mtu + ether_hdr_len)
>>> +            dev_info->rx_offload_capa |= DEV_RX_OFFLOAD_JUMBO_FRAME;
>>> +    } else {
>>> +        dev_info->rx_offload_capa |= DEV_RX_OFFLOAD_JUMBO_FRAME;
>>> +    }
>>> +
>>
>> As I wrote for v3, hw->max_mtu already calculated taking VIRTIO_NET_F_MTU
>> into account. If VIRTIO_NET_F_MTU is not set, hw->max_mtu is equal to
>> "VIRTIO_MAX_RX_PKTLEN - ETHER_HDR_LEN - VLAN_TAG_LEN - hw->vtnet_hdr_size".
>> i.e. "hw->max_mtu + ether_hdr_len" equal to VIRTIO_MAX_RX_PKTLEN which is
>> larger or equal to rxmode.max_rx_pkt_len.
>> So, there is no need to check for VIRTIO_NET_F_MTU here. You may just perform
>> same check for both cases.
> 
> I should read more carefully :). I think I get what you mean now.
> hw->max_mtu already includes ether_hdr_len, so it doesn't need
> re-calculation here. And in case of VIRTIO_NET_F_MTU the mtu is
> checked during feature negotiation. 
> So it basically boils down to        if (dev->data->dev_conf.rxmode.max_rx_pkt_len <= w->max_mtu)
>                dev_info->rx_offload_capa |= DEV_RX_OFFLOAD_JUMBO_FRAME;
> 

'hw->max_mtu' does not include 'ether_hdr_len'. But yes, VIRTIO_NET_F_MTU
already checked during negotiation. So, it will be:

if (dev->data->dev_conf.rxmode.max_rx_pkt_len <= hw->max_mtu + ether_hdr_len)
    dev_info->rx_offload_capa |= DEV_RX_OFFLOAD_JUMBO_FRAME;

>> This doesn't give any performance or so, but will simplify the code.
> 
> Thanks for the review!
> 
> regards,
> Jens
>