Re: [RFC PATCH v3 02/10] net: Pad short frames to minimum size before send from SLiRP/TAP

[Peter Maydell]

On Thu, 11 Mar 2021 at 09:58, Bin Meng <bmeng.cn@gmail.com> wrote:
>
> On Thu, Mar 11, 2021 at 5:43 PM Peter Maydell <peter.maydell@linaro.org> 
> wrote:
> >
> > On Thu, 11 Mar 2021 at 03:01, Jason Wang <jasowang@redhat.com> wrote:
> > > And after a discussion 10 years ago [1]. Michael (cced) seems to want to
> > > keep the padding logic in the NIC itself (probably with a generic helper
> > > in the core). Since 1) the padding is only required for ethernet 2)
> > > virito-net doesn't need that (it can pass incomplete packet by design).
> >
> > Like I said, we need to decide; either:
> >
> >  (1) we do want to support short packets in the net core:
> > every sender needs to either pad, or to have some flag to say
> > "my implementation can't pad, please can the net core do it for me",
> > unless they are deliberately sending a short packet. Every
> > receiver needs to be able to cope with short packets, at least
> > in the sense of not crashing (they should report them as a rx
> > error if they have that kind of error reporting status register).
> > I think we have mostly implemented our NIC models this way.
> >
> >  (2) we simply don't support short packets in the net core:
> > nobody (not NICs, not network backends) needs to pad, because
> > they can rely on the core to do it. Some existing senders and
> > receivers may have now-dead code to do their own padding which
> > could be removed.
> >
> > MST is advocating for (1) in that old thread. That's a coherent
> > position.
>
> But it's a wrong approach. As Edgar and Stefan also said in the old
> discussion thread, padding in the RX is wrong as real world NICs don't
> do this.

Neither option (1) nor option (2) involve padding in RX.

Option (1) is:
 * no NIC implementation pads on TX, except as defined
   by whatever NIC-specific config registers or h/w behaviour
   might require (ie if the guest wants to send a short packet
   it can do that)
 * non-NIC sources like slirp need to pad on TX unless they're
   deliberately trying to send a short packet
 * all receivers of packets need to cope with being given a
   short packet; this is usually going to mean "flag it to the
   guest as an RX error", but exact behaviour is NIC-dependent

Option (2) is:
 * the net core code pads any packet that goes through it
 * no NIC implementation needs to pad on TX (it is harmless if they do)
 * non-NIC sources don't need to pad on TX
 * no receivers of packets need to cope with being given short packets

Option 1 is what the real world does. Option 2 is a simplification
which throws away the ability to emulate handling of short packets,
in exchange for not having to sort out senders like slirp and not
having to be so careful about short-packet handling in NIC models.

If MST is correct that some use cases require short-packet support,
then we need to go for option 1, I think.

thanks
-- PMM