On Sun, Jul 2, 2023 at 9:35 AM Bin Meng <bmeng.cn@gmail.com> wrote:
>
> On Sun, Jun 25, 2023 at 9:59 AM Bin Meng <bmeng@tinylab.org> 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 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.
> >
> > Only the first 4 patches of the v5 series [1] were applied in QEMU 6.0,
> > and the reset was said to be queued for 6.1 but for some reason they
> > never landed in QEMU mainline.
> >
> > Hopefully this series will make it for QEMU 8.1.
> >
> > [1] https://lore.kernel.org/qemu-devel/859cd26a-feb2-ed62-98d5-764841a468cf@redhat.com/
> >
> > Changes in v7:
> > - new patch: "hw/net: ftgmac100: Drop the small packet check in the receive path"
> >
>
> Ping?
I've queued this series.
Thanks
>