netdev - Re: [PATCH net-next] virtio-net: restrict build

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Message-ID: <20210420215307.GA103196@roeck-us.net>
Date:   Tue, 20 Apr 2021 14:53:07 -0700
From:   Guenter Roeck <linux@...ck-us.net>
To:     Eric Dumazet <eric.dumazet@...il.com>
Cc:     "David S . Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        netdev <netdev@...r.kernel.org>,
        Eric Dumazet <edumazet@...gle.com>,
        Xuan Zhuo <xuanzhuo@...ux.alibaba.com>,
        Jason Wang <jasowang@...hat.com>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        virtualization@...ts.linux-foundation.org
Subject: Re: [PATCH net-next] virtio-net: restrict build_skb() use to some
 arches

On Tue, Apr 20, 2021 at 01:01:44PM -0700, Eric Dumazet wrote:
> From: Eric Dumazet <edumazet@...gle.com>
> 
> build_skb() is supposed to be followed by
> skb_reserve(skb, NET_IP_ALIGN), so that IP headers are word-aligned.
> (Best practice is to reserve NET_IP_ALIGN+NET_SKB_PAD, but the NET_SKB_PAD
> part is only a performance optimization if tunnel encaps are added.)
> 
> Unfortunately virtio_net has not provisioned this reserve.
> We can only use build_skb() for arches where NET_IP_ALIGN == 0
> 
> We might refine this later, with enough testing.
> 
> Fixes: fb32856b16ad ("virtio-net: page_to_skb() use build_skb when there's sufficient tailroom")
> Signed-off-by: Eric Dumazet <edumazet@...gle.com>
> Reported-by: Guenter Roeck <linux@...ck-us.net>
> Cc: Xuan Zhuo <xuanzhuo@...ux.alibaba.com>
> Cc: Jason Wang <jasowang@...hat.com>
> Cc: "Michael S. Tsirkin" <mst@...hat.com>
> Cc: virtualization@...ts.linux-foundation.org

Tested-by: Guenter Roeck <linux@...ck-us.net>

on alpha, sh4 (little endian).

Thanks!

Guenter