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

[Jens Freimann <jfreimann@redhat.com>]

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;

>This doesn't give any performance or so, but will simplify the code.

Thanks for the review!

regards,
Jens