[PATCH net-next] virtio-net: restrict build_skb() use to some arches

[PATCH net-next] virtio-net: restrict build_skb() use to some arches

[Eric Dumazet]

From: Eric Dumazet <edumazet@google.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@google.com>
Reported-by: Guenter Roeck <linux@roeck-us.net>
Cc: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
Cc: Jason Wang <jasowang@redhat.com>
Cc: "Michael S. Tsirkin" <mst@redhat.com>
Cc: virtualization@lists.linux-foundation.org
---
 drivers/net/virtio_net.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
index 2e28c04aa6351d2b4016f7d277ce104c4970069d..74d2d49264f3f3b7039be70331d4a44c53b8cc28 100644
--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c
@@ -416,7 +416,7 @@ static struct sk_buff *page_to_skb(struct virtnet_info *vi,
 
 	shinfo_size = SKB_DATA_ALIGN(sizeof(struct skb_shared_info));
 
-	if (len > GOOD_COPY_LEN && tailroom >= shinfo_size) {
+	if (!NET_IP_ALIGN && len > GOOD_COPY_LEN && tailroom >= shinfo_size) {
 		skb = build_skb(p, truesize);
 		if (unlikely(!skb))
 			return NULL;
-- 
2.31.1.368.gbe11c130af-goog

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Re: [PATCH net-next] virtio-net: restrict build_skb() use to some arches

[Guenter Roeck]

On Tue, Apr 20, 2021 at 01:01:44PM -0700, Eric Dumazet wrote:
> From: Eric Dumazet <edumazet@google.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@google.com>
> Reported-by: Guenter Roeck <linux@roeck-us.net>
> Cc: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
> Cc: Jason Wang <jasowang@redhat.com>
> Cc: "Michael S. Tsirkin" <mst@redhat.com>
> Cc: virtualization@lists.linux-foundation.org

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

on alpha, sh4 (little endian).

Thanks!

Guenter
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Re: [PATCH net-next] virtio-net: restrict build_skb() use to some arches

[Jason Wang]

在 2021/4/21 上午4:01, Eric Dumazet 写道:
> From: Eric Dumazet <edumazet@google.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@google.com>
> Reported-by: Guenter Roeck <linux@roeck-us.net>
> Cc: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
> Cc: Jason Wang <jasowang@redhat.com>
> Cc: "Michael S. Tsirkin" <mst@redhat.com>
> Cc: virtualization@lists.linux-foundation.org


Acked-by: Jason Wang <jasowang@redhat.com>


> ---
>   drivers/net/virtio_net.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index 2e28c04aa6351d2b4016f7d277ce104c4970069d..74d2d49264f3f3b7039be70331d4a44c53b8cc28 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -416,7 +416,7 @@ static struct sk_buff *page_to_skb(struct virtnet_info *vi,
>   
>   	shinfo_size = SKB_DATA_ALIGN(sizeof(struct skb_shared_info));
>   
> -	if (len > GOOD_COPY_LEN && tailroom >= shinfo_size) {
> +	if (!NET_IP_ALIGN && len > GOOD_COPY_LEN && tailroom >= shinfo_size) {
>   		skb = build_skb(p, truesize);
>   		if (unlikely(!skb))
>   			return NULL;

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