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

[Bin Meng]

On Thu, Mar 11, 2021 at 11:01 AM Jason Wang <jasowang@redhat.com> wrote:
>
>
> On 2021/3/9 6:13 下午, Peter Maydell wrote:
> > On Tue, 9 Mar 2021 at 09:01, Bin Meng <bmeng.cn@gmail.com> wrote:
> >> Hi Jason,
> >>
> >> On Tue, Mar 9, 2021 at 5:00 PM Bin Meng <bmeng.cn@gmail.com> wrote:
> >>> Hi Jason,
> >>>
> >>> On Tue, Mar 9, 2021 at 4:57 PM Jason Wang <jasowang@redhat.com> wrote:
> >>>>
> >>>> On 2021/3/9 4:35 下午, Bin Meng wrote:
> >>>>> Hi Jason,
> >>>>>
> >>>>> On Tue, Mar 9, 2021 at 4:23 PM Jason Wang <jasowang@redhat.com> wrote:
> >>>>>> On 2021/3/8 6:22 下午, Peter Maydell wrote:
> >>>>>>> I think the key thing we need to do here is make a decision
> >>>>>>> and be clear about what we're doing. There are three options
> >>>>>>> I can see:
> >>>>>>>
> >>>>>>> (1) we say that the net API demands that backends pad
> >>>>>>> packets they emit to the minimum ethernet frame length
> >>>>>>> unless they specifically are intending to emit a short frame,
> >>>>>>> and we fix any backends that don't comply (or equivalently,
> >>>>>>> add support in the core code for a backend to mark itself
> >>>>>>> as "I don't pad; please do it for me").
> >>>>>>>
> >>>>>>> (2) we say that the networking subsystem doesn't support
> >>>>>>> short packets, and just have the common code always enforce
> >>>>>>> padding short frames to the minimum length somewhere between
> >>>>>>> when it receives a packet from a backend and passes it to
> >>>>>>> a NIC model.
> >>>>>>>
> >>>>>>> (3) we say that it's the job of the NIC models to pad
> >>>>>>> short frames as they see them coming in.
> >>>>>> I'm not sure how much value we can gain from (1). So (2) looks better 
> >>>>>> to me.
> >>>>>>
> >>>>>> Bin or Philippe, want to send a new version?
> >>>>>>
> >>>>> I think this series does what (2) asks for. Or am I missing anything?
> >>>>
> >>>> It only did the padding for user/TAP.
> >> (hit send too soon ...)
> >>
> >> Ah, so we want this:
> >>
> >> if (sender->info->type != NET_CLIENT_DRIVER_NIC)
> >>
> >> correct?
> > No, option (2) is "always pad short packets regardless of
> > sender->info->type". Even if a NIC driver sends out a short
> > packet, we want to pad it, because we might be feeding it to
> > something that assumes it does not see short packets.
> >
> > thanks
> > -- PMM
>
>
> So I'm not sure this is correct. There're NIC that has its own logic
> that choose whether to pad the frame during TX (e.g e1000).

Yes, that's why I mentioned in v2's cover letter that we should
probably only do the padding for SLiRP and TAP. For NIC models, we can
still support sending short frames in QEMU.

>
> 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).
>

I did read this discussion before working on this patch series.
Providing a helper for NICs to call does NOT fix the issue for SLiRP
and TAP.

> Thanks
>
> [1]
> https://patchwork.ozlabs.org/project/qemu-devel/patch/1284842625-13920-1-git-send-email-stefanha@linux.vnet.ibm.com/
>

Regards,
Bin