Re: [PATCH v2] hw/net: fsl_etsec: Reverse the RCTRL.RSF logic

[Peter Maydell]

On Tue, 9 Feb 2021 at 01:22, Bin Meng <bmeng.cn@gmail.com> wrote:
>
> From: Bin Meng <bin.meng@windriver.com>
>
> Per MPC8548ERM [1] chapter 14.5.3.4.1:
>
> When RCTRL.RSF is 1, frames less than 64 bytes are accepted upon
> a DA match. But currently QEMU does the opposite.
>
> When RCTRL.RSF is 0, short frames are silently dropped, however
> we cannot drop such frames in QEMU as of today, due to both slirp
> and tap networking do not pad short frames (e.g.: an ARP packet)
> to the minimum frame size of 60 bytes.
>
> If eTSEC is programmed to reject short frames, ARP requests will be
> dropped, preventing the guest from becoming visible on the network.
>
> The same issue was reported on e1000 and vmxenet3 before, see:
>
> commit 78aeb23eded2 ("e1000: Pad short frames to minimum size (60 bytes)")
> commit 40a87c6c9b11 ("vmxnet3: Pad short frames to minimum size (60 bytes)")
>
> Ideally this should be fixed on the slirp/tap networking side to
> pad short frames to the minimum frame length, but I am not sure
> whether that's doable.
>
> This commit reverses the RCTRL.RSF testing logic to match the spec.
> The log message is updated to mention the reject short frames
> functionality is unimplemented.
>
> [1] https://www.nxp.com/docs/en/reference-manual/MPC8548ERM.pdf
>
> Fixes: eb1e7c3e5146 ("Add Enhanced Three-Speed Ethernet Controller (eTSEC)")
> Signed-off-by: Bin Meng <bin.meng@windriver.com>


> -    if ((etsec->regs[RCTRL].value & RCTRL_RSF) && (size < 60)) {
> +    /*
> +     * Both slirp and tap networking do not pad short frames
> +     * (e.g.: an ARP packet) to the minimum frame size of 60 bytes.
> +     *
> +     * If eTSEC is programmed to reject short frames, ARP requests
> +     * will be dropped, preventing the guest from becoming visible
> +     * on the network.
> +     */
> +    if (!(etsec->regs[RCTRL].value & RCTRL_RSF) && (size < 60)) {
>          /* CRC is not in the packet yet, so short frame is below 60 bytes */
> -        RING_DEBUG("%s: Drop short frame\n", __func__);
> -        return -1;
> +        RING_DEBUG("%s: Drop short frame not implemented\n", __func__);
>      }

This change is doing two things at once.

One of them is an entirely uncontroversial bug fix: we
got the sense of the RCTRL_RSF test the wrong way round.

The other is different: it is working around a bug elsewhere in QEMU.

If there's a problem with packets that should not be short
frames being presented to ethernet devices as short frames,
please fix that bug at the source. I don't think we should
take any more device-model workarounds for it. We have lots
and lots of ethernet device models: it will be much more
effort to try to fix them all one by one as people encounter
this bug than it would be to just fix the code that's creating
bogus short frames.

David, could you drop this from your queue, please ?

thanks
-- PMM