Re: [dpdk-dev] Where is the padding code in DPDK?

[Sam <batmanustc@gmail.com>]

So, to be brief, rte_eth_rx_burst and rte_eth_tx_burst, just send mbuf,
will not do anything.
Is that right?

Wiles, Keith <keith.wiles@intel.com> 于2018年11月15日周四 上午12:19写道：

>
>
> > On Nov 14, 2018, at 4:51 AM, Morten Brørup <mb@smartsharesystems.com>
> wrote:
> >
> > Anatoly,
> >
> > This differs from the Linux kernel's behavior, where padding belongs in
> the NIC driver layer, not in the protocol layer. If you pass a runt frame
> (too short packet) to a Linux NIC driver's transmission function, the NIC
> driver (or NIC hardware) will pad the frame to make it valid. E.g. look at
> the rhine_start_tx() function in the kernel:
> https://elixir.bootlin.com/linux/v4.9.137/source/drivers/net/ethernet/via/via-rhine.c#L1800
>
> The PMD in DPDK rejects the frame or extend the number of bytes to send.
> Padding assumes you are zeroing out the packet to meet the NIC required
> length. In PMDs unless they are concerned with security they just make sure
> the number of bytes to be sent are correct for the hardware (60 bytes min).
> Most NICs can do this padding in hardware as the packet is sent.
>
> If we are talking about virtio and only talking to virtio software backend
> then you can send any size packet, but the stacks or code receiving the
> packet you need to make sure it does not throw the packet away because it
> is a runt packet. Most NICs throw away Runts and are never received to
> memory. In software based design like virtio you can do whatever you want
> in the length, but I would suggest following the Ethernet standard anyway.
>
> Now some stacks or code (like Pktgen) assume the hardware will append the
> CRC (4 bytes) and this means the application needs to at least do 60 byte
> frames for the PMD, unless you know the hardware will do the right thing.
> The challenge is that applications in DPDK do not know the details of the
> NIC at that level and should always assume the packet being sent and
> received are valid Ethernet frames. This means at lease 60 bytes as all
> NICs add the CRC now a days and not all of them adjust the size of the
> frame.
>
> If you do not send the PMD a 60 byte frame then you are expecting the NIC
> to handle the padding and appending the CRC or at least expecting the PMD
> to adjust the size, which I know is not in all PMDs or from my dealing with
> writing Pktgen for DPDK.
>
> If you are expecting DPDK PMDs to be Linux drivers then you need to adjust
> your thinking and only send the PMD 60 bytes at least. Unless you want to
> modify all of the PMDs to force the size to 60bytes, then I have no
> objection to that patch just need to get all of the PMDs maintainers to
> agree with your patch.
>
> On RX frames of less then 64 bytes (with CRC) are runts and most NICs
> today will not receive these frames unless you program the hardware to do
> so. ‘In my day’ :-) we had collision on the wire which created a huge
> amount of fragments or Runts, today is not the case with point-to-point
> links we have today.
>
> >
> > If DPDK does not pad short frames passed to the egress function of the
> NIC drivers, it should be noted in the documentation - this is not the
> expected behavior by protocol developers.
> >
> > Or even better: The NIC hardware (or driver) should ensure padding,
> possibly considering it a TX Offload feature. Generating packets shorter
> than 60 bytes data is common - just consider the amount of TCP ACK packets,
> which are typically only 14 + 20 + 20 = 54 bytes (incl. the 14 byte
> Ethernet header).
> >
> >
> > Med venlig hilsen / kind regards
> > - Morten Brørup
> >
> >> -----Original Message-----
> >> From: dev [mailto:dev-bounces@dpdk.org] On Behalf Of Burakov, Anatoly
> >> Sent: Wednesday, November 14, 2018 11:18 AM
> >> To: Sam
> >> Cc: dev@dpdk.org
> >> Subject: Re: [dpdk-dev] Where is the padding code in DPDK?
> >>
> >> On 14-Nov-18 5:45 AM, Sam wrote:
> >>> OK, then shortly speaking, DPDK will NOT care about padding.
> >>> NIC will care about padding while send and recv with NIC.
> >>> kernel will care about while send and recv with vhostuser port.
> >>>
> >>> Is that right?
> >>
> >> I cannot speak for virtio/vhost user since i am not terribly familiar
> >> with them. For regular packets, generally speaking, packets shorter
> >> than
> >> 60 bytes are invalid. Whether DPDK does or does not care about padding
> >> is irrelevant, because *you* are attempting to transmit packets that
> >> are
> >> not valid. You shouldn't rely on this behavior.
> >>
> >>>
> >>>
> >>> Burakov, Anatoly <anatoly.burakov@intel.com
> >>> <mailto:anatoly.burakov@intel.com>> 于2018年11月13日周二 下午5:29写道：
> >>>
> >>>    On 13-Nov-18 7:16 AM, Sam wrote:
> >>>> Hi all,
> >>>>
> >>>> As we know, ethernet frame must longer then 64B.
> >>>>
> >>>> So if I create rte_mbuf and fill it with just 60B data, will
> >>>> rte_eth_tx_burst add padding data, let the frame longer then
> >> 64B
> >>>>
> >>>> If it does, where is the code?
> >>>>
> >>>
> >>>    Others can correct me if i'm wrong here, but specifically in case
> >> of
> >>>    64-byte packets, these are the shortest valid packets that you
> >> can
> >>>    send,
> >>>    and a 64-byte packet will actually carry only 60 bytes' worth of
> >> packet
> >>>    data, because there's a 4-byte CRC frame at the end (see Ethernet
> >> frame
> >>>    format). If you enabled CRC offload, then your NIC will append
> >> the 4
> >>>    bytes at transmit. If you haven't, then it's up to each
> >> individual
> >>>    driver/NIC to accept/reject such a packet because it can rightly
> >> be
> >>>    considered malformed.
> >>>
> >>>    In addition, your NIC may add e.g. VLAN tags or other stuff,
> >> again
> >>>    depending on hardware offloads that you have enabled in your TX
> >>>    configuration, which may push the packet size beyond 64 bytes
> >> while
> >>>    having only 60 bytes of actual packet data.
> >>>
> >>>    --
> >>>    Thanks,
> >>>    Anatoly
> >>>
> >>
> >>
> >> --
> >> Thanks,
> >> Anatoly
> >
>
> Regards,
> Keith
>
>