What is the general design principle for QEMU's handling of "short"
packets, which are less than the 60 byte ethernet frame minimum?
In commit 969e50b61a285b0 ("net: Pad short frames to minimum size
before sending from SLiRP/TAP") we made the slirp and tap backends
add padding to short packets before feeding them into the net
code via qemu_send_packet() and friends, with the idea that we could
then remove (often buggy) code from network device emulation that
tried to handle the short-packet in its receive path.
In commit 79b6a985 we realized we also needed to do this for the vmnet
backend. The issue https://gitlab.com/qemu-project/qemu/-/work_items/2362
reports that we also need to do this for socket and dgram and similar
backends.
I'm trying to identify what the underlying logic here is. My current
best guess is:
- we should always pad before feeding in a packet which is going
to be sent to an ethernet device model (unless that device model
has explicitly opted in via do_not_pad, e.g. because it really does
want to faithfully model the card's handling of short packets
and it's gone to the trouble of getting it right)
- we should not pad before feeding in a packet which is going to
be sent to a network backend (i.e. where the guest is sending out
to slirp/tap/etc)
Could we do this by having qemu_send_packet() etc always pad, and
have the slirp/tap/etc backends set the do_not_pad flag on their
NetClientState structs to suppress this? Would that be better than
having all the backends do "and pad this" for their sending code?
At the moment the only user of do_not_pad is the virtio-net device.
commit d4c6293041 just says "there is no need to pad" and doesn't
give rationale. Why is this particular network device a special case?
Are there any special cases where code calls qemu_send_packet
or qemu_sendv_packet which aren't either "a network device"
or "a backend"? There's net/hub.c -- I guess that should set
do_not_pad for its inputs, so it faithfully passes through
short packets and lets the destinations decide whether they want
them padded or not. Anything else?
-- PMM
