netdev - Re: [PATCH net-next] net: mvpp2: increase MTU limit when XDP enabled

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Date:   Tue, 5 Jan 2021 18:24:37 +0100
From:   Sven Auhagen <sven.auhagen@...eatech.de>
To:     Marek Behún <kabel@...nel.org>
Cc:     netdev@...r.kernel.org, davem@...emloft.net,
        Jakub Kicinski <kuba@...nel.org>,
        Matteo Croce <mcroce@...rosoft.com>,
        Lorenzo Bianconi <lorenzo@...nel.org>
Subject: Re: [PATCH net-next] net: mvpp2: increase MTU limit when XDP enabled

On Tue, Jan 05, 2021 at 06:19:21PM +0100, Marek Behún wrote:
> Currently mvpp2_xdp_setup won't allow attaching XDP program if
>   mtu > ETH_DATA_LEN (1500).
> 
> The mvpp2_change_mtu on the other hand checks whether
>   MVPP2_RX_PKT_SIZE(mtu) > MVPP2_BM_LONG_PKT_SIZE.
> 
> These two checks are semantically different.
> 
> Moreover this limit can be increased to MVPP2_MAX_RX_BUF_SIZE, since in
> mvpp2_rx we have
>   xdp.data = data + MVPP2_MH_SIZE + MVPP2_SKB_HEADROOM;
>   xdp.frame_sz = PAGE_SIZE;
> 
> Change the checks to check whether
>   mtu > MVPP2_MAX_RX_BUF_SIZE

Hello Marek,

in general, XDP is based on the model, that packets are not bigger than 1500.
I am not sure if that has changed, I don't believe Jumbo Frames are upstreamed yet.
You are correct that the MVPP2 driver can handle bigger packets without a problem but
if you do XDP redirect that won't work with other drivers and your packets will disappear.

Best
Sven

> 
> Signed-off-by: Marek Behún <kabel@...nel.org>
> Cc: Sven Auhagen <sven.auhagen@...eatech.de>
> Cc: Matteo Croce <mcroce@...rosoft.com>
> Cc: Lorenzo Bianconi <lorenzo@...nel.org>
> ---
>  drivers/net/ethernet/marvell/mvpp2/mvpp2_main.c | 13 +++++++------
>  1 file changed, 7 insertions(+), 6 deletions(-)
> 
> diff --git a/drivers/net/ethernet/marvell/mvpp2/mvpp2_main.c b/drivers/net/ethernet/marvell/mvpp2/mvpp2_main.c
> index afdd22827223..65490a0eb657 100644
> --- a/drivers/net/ethernet/marvell/mvpp2/mvpp2_main.c
> +++ b/drivers/net/ethernet/marvell/mvpp2/mvpp2_main.c
> @@ -4623,11 +4623,12 @@ static int mvpp2_change_mtu(struct net_device *dev, int mtu)
>  		mtu = ALIGN(MVPP2_RX_PKT_SIZE(mtu), 8);
>  	}
>  
> +	if (port->xdp_prog && mtu > MVPP2_MAX_RX_BUF_SIZE) {
> +		netdev_err(dev, "Illegal MTU value %d for XDP mode\n", mtu);
> +		return -EINVAL;
> +	}
> +
>  	if (MVPP2_RX_PKT_SIZE(mtu) > MVPP2_BM_LONG_PKT_SIZE) {
> -		if (port->xdp_prog) {
> -			netdev_err(dev, "Jumbo frames are not supported with XDP\n");
> -			return -EINVAL;
> -		}
>  		if (priv->percpu_pools) {
>  			netdev_warn(dev, "mtu %d too high, switching to shared buffers", mtu);
>  			mvpp2_bm_switch_buffers(priv, false);
> @@ -4913,8 +4914,8 @@ static int mvpp2_xdp_setup(struct mvpp2_port *port, struct netdev_bpf *bpf)
>  	bool running = netif_running(port->dev);
>  	bool reset = !prog != !port->xdp_prog;
>  
> -	if (port->dev->mtu > ETH_DATA_LEN) {
> -		NL_SET_ERR_MSG_MOD(bpf->extack, "XDP is not supported with jumbo frames enabled");
> +	if (port->dev->mtu > MVPP2_MAX_RX_BUF_SIZE) {
> +		NL_SET_ERR_MSG_MOD(bpf->extack, "MTU too large for XDP");
>  		return -EOPNOTSUPP;
>  	}
>  
> -- 
> 2.26.2
>