Re: [Qemu-devel] [PATCH] e1000: Pad short frames to minimum size (60 bytes)

[Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>]

On Mon, Sep 20, 2010 at 10:42:31AM +0200, Kevin Wolf wrote:
> Am 18.09.2010 23:12, schrieb Stefan Hajnoczi:
> > On Sat, Sep 18, 2010 at 9:57 PM, Hervé Poussineau <hpoussin@reactos.org> wrote:
> >> Another patch creating ARP replies at least 64 bytes long has been
> >> committed:
> >> http://git.savannah.gnu.org/cgit/qemu.git/commit/?id=dbf3c4b4baceb91eb64d09f787cbe92d65188813
> >>
> >> Does it fix your issue?
> > 
> > No I don't think so.  This is an e1000 issue, it will happen if you
> > use tap networking too.  The commit you linked to only affects slirp
> > and pads its ARP code.
> > 
> > I think there are two places where the minimum frame length can be enforced:
> > 1. The NIC emulation code.  This is currently how rtl8139, pcnet, and
> > ne2000 do it.  My patch adds the same for e1000.
> > 2. The net layer.  If we're emulating Ethernet then it would be
> > possible to pad to minimum frame length in common networking code
> > (net.c).
> 
> 3. The sender. I think it should be the sender's decision which packet
> he sends and there's no reason to manipulate it on its way to the guest.
> If the sender sends too short packets, this is where the bug is.
> 
> Actually, instead of padding the packet we should already drop it in the
> device model if RCTL.SBP = 0. Does a real Solaris work when it receives
> the same packet?
> 
> On the other hand, it seems that we're missing the padding where it
> actually belongs: when sending packets with TCTL.PSP = 1. Did you send
> the ARP packet from another qemu instance? If so, this might be the real
> reason.

No, I brought up the tap interface on the host and tried to ping the
guest directly.  This caused the host to send an ARP request over the
tap interface.  It was not padded.

Perhaps Linux expects the NIC to deal with padding transmit frames but
tap does not do this.  That's just my theory though from the experience
that real NICs often do pad to minimum size.  Does anyone know the
definitive answer?

Stefan