Re: [PATCH v5 00/12] net: Pad short frames for network backends

[Jason Wang <jasowang@redhat.com>]

在 2021/3/17 下午2:26, Bin Meng 写道:
> The minimum Ethernet frame length is 60 bytes. For short frames with
> smaller length like ARP packets (only 42 bytes), on a real world NIC
> it can choose either padding its length to the minimum required 60
> bytes, or sending it out directly to the wire. Such behavior can be
> hardcoded or controled by a register bit. Similarly on the receive
> path, NICs can choose either dropping such short frames directly or
> handing them over to software to handle.
>
> On the other hand, for the network backends like SLiRP/TAP, they
> don't expose a way to control the short frame behavior. As of today
> they just send/receive data from/to the other end connected to them,
> which means any sized packet is acceptable. So they can send and
> receive short frames without any problem. It is observed that ARP
> packets sent from SLiRP/TAP are 42 bytes, and SLiRP/TAP just send
> these ARP packets to the other end which might be a NIC model that
> does not allow short frames to pass through.
>
> To provide better compatibility, for packets sent from QEMU network
> backends like SLiRP/TAP, we change to pad short frames before sending
> it out to the other end, if the other end does not forbid it via the
> nc->do_not_pad flag. This ensures a backend as an Ethernet sender
> does not violate the spec. But with this change, the behavior of
> dropping short frames from SLiRP/TAP interfaces in the NIC model
> cannot be emulated because it always receives a packet that is spec
> complaint. The capability of sending short frames from NIC models is
> still supported and short frames can still pass through SLiRP/TAP.
>
> This series should be able to fix the issue as reported with some
> NIC models before, that ARP requests get dropped, preventing the
> guest from becoming visible on the network. It was workarounded in
> these NIC models on the receive path, that when a short frame is
> received, it is padded up to 60 bytes.
>
> Changes in v5:
> - minor update on commit message
> - update the eth_pad_short_frame() comment
>
> Changes in v4:
> - change 'ethernet' to 'Ethernet'
> - do not inline the helper
> - check the padded buffer size to avoid buffer overflow
> - squash slirp/tap commits into one
>
> Changes in v3:
> - use 'without' instead of 'sans'
> - add a helper to pad short frames
> - add a comment to 'do_not_pad'
> - use the pad_short_frame() helper
>
> Bin Meng (12):
>    net: eth: Add a helper to pad a short Ethernet frame
>    net: Add a 'do_not_pad" to NetClientState
>    net: Pad short frames to minimum size before sending from SLiRP/TAP
>    hw/net: virtio-net: Initialize nc->do_not_pad to true
>    hw/net: e1000: Remove the logic of padding short frames in the receive
>      path
>    hw/net: vmxnet3: Remove the logic of padding short frames in the
>      receive path
>    hw/net: i82596: Remove the logic of padding short frames in the
>      receive path
>    hw/net: ne2000: Remove the logic of padding short frames in the
>      receive path
>    hw/net: pcnet: Remove the logic of padding short frames in the receive
>      path
>    hw/net: rtl8139: Remove the logic of padding short frames in the
>      receive path
>    hw/net: sungem: Remove the logic of padding short frames in the
>      receive path
>    hw/net: sunhme: Remove the logic of padding short frames in the
>      receive path
>
>   hw/net/e1000.c      | 11 +----------
>   hw/net/i82596.c     | 18 ------------------
>   hw/net/ne2000.c     | 12 ------------
>   hw/net/pcnet.c      |  9 ---------
>   hw/net/rtl8139.c    | 12 ------------
>   hw/net/sungem.c     | 14 --------------
>   hw/net/sunhme.c     | 11 -----------
>   hw/net/virtio-net.c |  4 ++++
>   hw/net/vmxnet3.c    | 10 ----------
>   include/net/eth.h   | 17 +++++++++++++++++
>   include/net/net.h   |  1 +
>   net/eth.c           | 17 +++++++++++++++++
>   net/slirp.c         | 10 ++++++++++
>   net/tap-win32.c     | 10 ++++++++++
>   net/tap.c           | 10 ++++++++++
>   15 files changed, 70 insertions(+), 96 deletions(-)


I've queued patch 1-4 for 6.0 and the reset for 6.1.

Thanks


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