Re: [PATCH] hw/net: fsl_etsec: Do not reject short frames

["Philippe Mathieu-Daudé" <philmd@redhat.com>]

Cc'ing libSLiRP.

On 2/8/21 7:41 AM, Bin Meng wrote:
> As of today 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 changes to codes to ignore the RCTRL_RSF setting and
> still allow receiving the short frame. The log message is updated
> to mention the reject short frames functionality is unimplemented.
> 
> Signed-off-by: Bin Meng <bin.meng@windriver.com>
> ---
> 
>  hw/net/fsl_etsec/rings.c | 11 +++++++++--
>  1 file changed, 9 insertions(+), 2 deletions(-)
> 
> diff --git a/hw/net/fsl_etsec/rings.c b/hw/net/fsl_etsec/rings.c
> index 121415a..503b4d3 100644
> --- a/hw/net/fsl_etsec/rings.c
> +++ b/hw/net/fsl_etsec/rings.c
> @@ -502,10 +502,17 @@ ssize_t etsec_rx_ring_write(eTSEC *etsec, const uint8_t *buf, size_t size)
>          return -1;
>      }
>  
> +    /*
> +     * 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__);
>      }
>  
>      rx_init_frame(etsec, buf, size);
>