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

[Bin Meng]

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:
> >>> On Mon, 8 Mar 2021 at 03:48, Jason Wang <jasowang@redhat.com> wrote:
> >>>> Do we need to care about other type of networking backends? E.g socket.
> >>>>
> >>>> Or at least we should keep the padding logic if we can't audit all of
> >>>> the backends.
> >>> 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 think (3) is pretty clearly the worst of these, since it
> >>> requires every NIC model to handle it; it has no advantages
> >>> over (2) that I can see. I don't have a strong take on whether
> >>> we'd rather have (1) or (2): it's a tradeoff between whether
> >>> we support modelling of short frames vs simplicity of code.
> >>> I'd just like us to be clear about what point or points in
> >>> the code have the responsibility for padding short frames.
> >>
> >> 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.

Ah, so we want this: