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From: "Michael S. Tsirkin" <mst@redhat.com>
To: Jason Wang <jasowang@redhat.com>
Cc: netdev@vger.kernel.org, Eli Cohen <elic@nvidia.com>,
	linux-kernel@vger.kernel.org,
	virtualization@lists.linux-foundation.org
Subject: Re: [PATCH net] virtio-net: enable big mode correctly
Date: Thu, 25 Nov 2021 02:00:26 -0500	[thread overview]
Message-ID: <20211125015532-mutt-send-email-mst@kernel.org> (raw)
In-Reply-To: <20211125060547.11961-1-jasowang@redhat.com>

On Thu, Nov 25, 2021 at 02:05:47PM +0800, Jason Wang wrote:
> When VIRTIO_NET_F_MTU feature is not negotiated, we assume a very
> large max_mtu. In this case, using small packet mode is not correct
> since it may breaks the networking when MTU is grater than
> ETH_DATA_LEN.
> 
> To have a quick fix, simply enable the big packet mode when
> VIRTIO_NET_F_MTU is not negotiated.

This will slow down dpdk hosts which disable mergeable buffers
and send standard MTU sized packets.

> We can do optimization on top.

I don't think it works like this, increasing mtu
from guest >4k never worked, we can't regress everyone's
performance with a promise to maybe sometime bring it back.

> Reported-by: Eli Cohen <elic@nvidia.com>
> Cc: Eli Cohen <elic@nvidia.com>
> Signed-off-by: Jason Wang <jasowang@redhat.com>
>
> ---
>  drivers/net/virtio_net.c | 7 ++++---
>  1 file changed, 4 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index 7c43bfc1ce44..83ae3ef5eb11 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -3200,11 +3200,12 @@ static int virtnet_probe(struct virtio_device *vdev)
>  		dev->mtu = mtu;
>  		dev->max_mtu = mtu;
>  
> -		/* TODO: size buffers correctly in this case. */
> -		if (dev->mtu > ETH_DATA_LEN)
> -			vi->big_packets = true;
>  	}
>  
> +	/* TODO: size buffers correctly in this case. */
> +	if (dev->max_mtu > ETH_DATA_LEN)
> +		vi->big_packets = true;
> +
>  	if (vi->any_header_sg)
>  		dev->needed_headroom = vi->hdr_len;
>  
> -- 
> 2.25.1

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Virtualization@lists.linux-foundation.org
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WARNING: multiple messages have this Message-ID (diff)
From: "Michael S. Tsirkin" <mst@redhat.com>
To: Jason Wang <jasowang@redhat.com>
Cc: virtualization@lists.linux-foundation.org,
	netdev@vger.kernel.org, linux-kernel@vger.kernel.org,
	Eli Cohen <elic@nvidia.com>
Subject: Re: [PATCH net] virtio-net: enable big mode correctly
Date: Thu, 25 Nov 2021 02:00:26 -0500	[thread overview]
Message-ID: <20211125015532-mutt-send-email-mst@kernel.org> (raw)
In-Reply-To: <20211125060547.11961-1-jasowang@redhat.com>

On Thu, Nov 25, 2021 at 02:05:47PM +0800, Jason Wang wrote:
> When VIRTIO_NET_F_MTU feature is not negotiated, we assume a very
> large max_mtu. In this case, using small packet mode is not correct
> since it may breaks the networking when MTU is grater than
> ETH_DATA_LEN.
> 
> To have a quick fix, simply enable the big packet mode when
> VIRTIO_NET_F_MTU is not negotiated.

This will slow down dpdk hosts which disable mergeable buffers
and send standard MTU sized packets.

> We can do optimization on top.

I don't think it works like this, increasing mtu
from guest >4k never worked, we can't regress everyone's
performance with a promise to maybe sometime bring it back.

> Reported-by: Eli Cohen <elic@nvidia.com>
> Cc: Eli Cohen <elic@nvidia.com>
> Signed-off-by: Jason Wang <jasowang@redhat.com>
>
> ---
>  drivers/net/virtio_net.c | 7 ++++---
>  1 file changed, 4 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index 7c43bfc1ce44..83ae3ef5eb11 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -3200,11 +3200,12 @@ static int virtnet_probe(struct virtio_device *vdev)
>  		dev->mtu = mtu;
>  		dev->max_mtu = mtu;
>  
> -		/* TODO: size buffers correctly in this case. */
> -		if (dev->mtu > ETH_DATA_LEN)
> -			vi->big_packets = true;
>  	}
>  
> +	/* TODO: size buffers correctly in this case. */
> +	if (dev->max_mtu > ETH_DATA_LEN)
> +		vi->big_packets = true;
> +
>  	if (vi->any_header_sg)
>  		dev->needed_headroom = vi->hdr_len;
>  
> -- 
> 2.25.1


  reply	other threads:[~2021-11-25  7:00 UTC|newest]

Thread overview: 25+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-11-25  6:05 [PATCH net] virtio-net: enable big mode correctly Jason Wang
2021-11-25  6:05 ` Jason Wang
2021-11-25  7:00 ` Michael S. Tsirkin [this message]
2021-11-25  7:00   ` Michael S. Tsirkin
2021-11-25  7:11   ` Jason Wang
2021-11-25  7:11     ` Jason Wang
2021-11-25  7:14     ` Michael S. Tsirkin
2021-11-25  7:14       ` Michael S. Tsirkin
2021-11-25  7:20       ` Jason Wang
2021-11-25  7:20         ` Jason Wang
2021-11-25  7:26         ` Michael S. Tsirkin
2021-11-25  7:26           ` Michael S. Tsirkin
2021-11-25  7:28           ` Jason Wang
2021-11-25  7:28             ` Jason Wang
2021-11-25  8:13             ` Michael S. Tsirkin
2021-11-25  8:13               ` Michael S. Tsirkin
2021-11-25  7:09 ` Eli Cohen
2021-11-25  7:15   ` Jason Wang
2021-11-25  7:15     ` Jason Wang
2021-11-25  7:20     ` Eli Cohen
2021-11-25  7:26       ` Jason Wang
2021-11-25  7:26         ` Jason Wang
2021-11-25  7:31         ` Michael S. Tsirkin
2021-11-25  7:31           ` Michael S. Tsirkin
2021-11-25  7:41         ` Eli Cohen

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