Re: [PATCH v4 03/12] net: Pad short frames to minimum size before sending from SLiRP/TAP

[Philippe Mathieu-Daudé]

On 3/16/21 1:04 PM, Bin Meng wrote:
> 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 commit 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.
> 
> The following 2 commits seem to be the one to workaround this issue
> in e1000 and vmxenet3 before, and should probably be reverted.
> 
>   commit 78aeb23eded2 ("e1000: Pad short frames to minimum size (60 bytes)")
>   commit 40a87c6c9b11 ("vmxnet3: Pad short frames to minimum size (60 bytes)")
> 
> Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
> 
> ---
> 
> Changes in v4:
> - squash slirp/tap commits into one
> 
> Changes in v3:
> - use the pad_short_frame() helper
> 
>  net/slirp.c     | 10 ++++++++++
>  net/tap-win32.c | 10 ++++++++++
>  net/tap.c       | 10 ++++++++++
>  3 files changed, 30 insertions(+)

Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>