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

[Bin Meng]

Hi Peter,

On Tue, Mar 9, 2021 at 6:13 PM Peter Maydell <peter.maydell@linaro.org> 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.

Then this is v1:
http://patchwork.ozlabs.org/project/qemu-devel/patch/1614333786-74258-2-git-send-email-bmeng.cn@gmail.com/

If yes, I will respin v3.

Regards,
Bin