Re: [dpdk-dev] [PATCH v6] net/virtio: add set_mtu in virtio

[Yuanhan Liu <yuanhan.liu@linux.intel.com>]

On Wed, Sep 21, 2016 at 06:45:05PM -0700, Stephen Hemminger wrote:
> On Thu, 22 Sep 2016 00:08:38 +0000
> "Dey, Souvik" <sodey@sonusnet.com> wrote:
> 
> > Answers inline.
> > 
> > --
> > Regards,
> > Souvik
> > 
> > -----Original Message-----
> > From: Stephen Hemminger [mailto:stephen@networkplumber.org] 
> > Sent: Wednesday, September 21, 2016 7:22 PM
> > To: Dey, Souvik <sodey@sonusnet.com>
> > Cc: mark.b.kavanagh@intel.com; yuanhan.liu@linux.intel.com; dev@dpdk.org
> > Subject: Re: [PATCH v6] net/virtio: add set_mtu in virtio
> > 
> > On Wed, 21 Sep 2016 19:11:47 -0400
> > Dey <sodey@sonusnet.com> wrote:
> > 
> > >  
> > > +
> > > +#define VLAN_TAG_SIZE           4    /* 802.3ac tag (not DMA'd) */
> > > +
> > > +static int virtio_mtu_set(struct rte_eth_dev *dev, uint16_t mtu) {
> > > +       struct rte_eth_dev_info dev_info;
> > > +       uint32_t ether_hdr_len = ETHER_HDR_LEN + ETHER_CRC_LEN + VLAN_TAG_SIZE;
> > > +       uint32_t frame_size = mtu + ether_hdr_len;
> > > +
> > > +       virtio_dev_info_get(dev, &dev_info);
> > > +
> > > +       if (mtu < ETHER_MIN_MTU || frame_size > dev_info.max_rx_pktlen) {
> > > +               PMD_INIT_LOG(ERR, "MTU should be between %d and %d\n",
> > > +                               ETHER_MIN_MTU,
> > > +                               (dev_info.max_rx_pktlen - ether_hdr_len));
> > > +               return -EINVAL;
> > > +       }
> > > +       return 0;
> > > +}  
> > 
> > I am fine with the general idea of this patch but:
> >   1. Calling virtio_dev_info_get is needlessly wasteful when all you want
> >      is to access the max packet length. Since max_rx_pktlen is always
> >      VIRTIO_MAX_RX_PKTLEN, please just use that.
> > [Dey, Souvik] I am using the virtio_dev_info_get as in future can/may support the max_rx_pktlen as a variable to be set by  the application. This will keep the changes future proof. As we need to support till 65535 instead of 9728 as the linux does.
> 
> Fine, then just dereference hw->rx_max_pktlen. Driver code can/should reference
> its own data directly.

Dey, maybe you could just use VIRTIO_MAX_RX_PKTLEN here, like what you
did in early versions.

> > 
> >   2. Defining VLAN_TAG_SIZE is irrelevant if doing vlan offload.
> > [Dey, Souvik] vlan offload is not mandatory. Se again still have vlan being sent up to the application. In that case we need to consider the vlan length in the Ethernet size.
> 
> The code needs to handle both vlan offload (or not), correctly. You are assuming
> the worst case here.

I think we are fine here to assume worst case.

> > 
> >   3. Virtio doesn't insert CRC, therefore CRC_LEN is irrelevant
> > [Dey, Souvik] I am not sure of this. Mark commented earlier to consider this length too. Mark what do you suggest ?
> 
> Actually, the thing that matters is the size of the merge rx buf header, not the CRC.

Right.

	--yliu
> 
> The patch is still buggy.
>