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Date:	Mon, 28 Jun 2010 15:19:41 -0700
From:	Sridhar Samudrala <sri@...ibm.com>
To:	"Michael S. Tsirkin" <mst@...hat.com>
Cc:	Aristeu Rozanski <arozansk@...hat.com>,
	Herbert Xu <herbert.xu@...hat.com>,
	Juan Quintela <quintela@...hat.com>,
	"David S. Miller" <davem@...hat.com>, kvm@...r.kernel.org,
	virtualization@...ts.osdl.org, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org, ykaul@...hat.com, markmc@...hat.com
Subject: Re: [PATCHv2] vhost-net: add dhclient work-around from userspace

On Mon, 2010-06-28 at 13:08 +0300, Michael S. Tsirkin wrote:
> Userspace virtio server has the following hack
> so guests rely on it, and we have to replicate it, too:
> 
> Use port number to detect incoming IPv4 DHCP response packets,
> and fill in the checksum for these.
> 
> The issue we are solving is that on linux guests, some apps
> that use recvmsg with AF_PACKET sockets, don't know how to
> handle CHECKSUM_PARTIAL;
> The interface to return the relevant information was added
> in 8dc4194474159660d7f37c495e3fc3f10d0db8cc,
> and older userspace does not use it.
> One important user of recvmsg with AF_PACKET is dhclient,
> so we add a work-around just for DHCP.
> 
> Don't bother applying the hack to IPv6 as userspace virtio does not
> have a work-around for that - let's hope guests will do the right
> thing wrt IPv6.
> 
> Signed-off-by: Michael S. Tsirkin <mst@...hat.com>
> ---
> 
> Dave, I'm going to put this patch on the vhost tree,
> no need for you to bother merging it - you'll get
> it with a pull request.
> 
> 
>  drivers/vhost/net.c |   44 +++++++++++++++++++++++++++++++++++++++++++-
>  1 files changed, 43 insertions(+), 1 deletions(-)
> 
> diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
> index cc19595..03bba6a 100644
> --- a/drivers/vhost/net.c
> +++ b/drivers/vhost/net.c
> @@ -24,6 +24,10 @@
>  #include <linux/if_tun.h>
>  #include <linux/if_macvlan.h>
> 
> +#include <linux/ip.h>
> +#include <linux/udp.h>
> +#include <linux/netdevice.h>
> +
>  #include <net/sock.h>
> 
>  #include "vhost.h"
> @@ -186,6 +190,44 @@ static void handle_tx(struct vhost_net *net)
>  	unuse_mm(net->dev.mm);
>  }
> 
> +static int peek_head(struct sock *sk)

This routine is doing more than just peeking the head of sk's receive
queue. May be this should be named similar to what qemu calls
'work_around_broken_dhclient()'
> +{
> +	struct sk_buff *skb;
> +
> +	lock_sock(sk);
> +	skb = skb_peek(&sk->sk_receive_queue);
> +	if (unlikely(!skb)) {
> +		release_sock(sk);
> +		return 0;
> +	}
> +	/* Userspace virtio server has the following hack so
> +	 * guests rely on it, and we have to replicate it, too: */
> +	/* Use port number to detect incoming IPv4 DHCP response packets,
> +	 * and fill in the checksum. */
> +
> +	/* The issue we are solving is that on linux guests, some apps
> +	 * that use recvmsg with AF_PACKET sockets, don't know how to
> +	 * handle CHECKSUM_PARTIAL;
> +	 * The interface to return the relevant information was added in
> +	 * 8dc4194474159660d7f37c495e3fc3f10d0db8cc,
> +	 * and older userspace does not use it.
> +	 * One important user of recvmsg with AF_PACKET is dhclient,
> +	 * so we add a work-around just for DHCP. */
> +	if (skb->ip_summed == CHECKSUM_PARTIAL &&
> +	    skb_headlen(skb) >= skb_transport_offset(skb) +
> +				sizeof(struct udphdr) &&
> +	    udp_hdr(skb)->dest == htons(68) &&
> +	    skb_network_header_len(skb) >= sizeof(struct iphdr) &&
> +	    ip_hdr(skb)->protocol == IPPROTO_UDP &&
> +	    skb->protocol == htons(ETH_P_IP)) {

Isn't it more logical to check for skb->protocol, followed by ip_hdr and
then udp_hdr?

> +		skb_checksum_help(skb);
> +		/* Restore ip_summed value: tun passes it to user. */
> +		skb->ip_summed = CHECKSUM_PARTIAL;
> +	}
> +	release_sock(sk);
> +	return 1;
> +}
> +
>  /* Expects to be always run from workqueue - which acts as
>   * read-size critical section for our kind of RCU. */
>  static void handle_rx(struct vhost_net *net)
> @@ -222,7 +264,7 @@ static void handle_rx(struct vhost_net *net)
>  	vq_log = unlikely(vhost_has_feature(&net->dev, VHOST_F_LOG_ALL)) ?
>  		vq->log : NULL;
> 
> -	for (;;) {
> +	while (peek_head(sock->sk)) {
>  		head = vhost_get_vq_desc(&net->dev, vq, vq->iov,
>  					 ARRAY_SIZE(vq->iov),
>  					 &out, &in,

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