Message-ID: <20130815194420.GA11999@redhat.com>
Date:	Thu, 15 Aug 2013 22:44:20 +0300
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Vlad Yasevich <vyasevic@...hat.com>
Cc:	netdev@...r.kernel.org
Subject: Re: [PATCH v3 1/2] macvtap: include all checksum offloads in
 TUN_OFFLOAD mask

On Thu, Aug 15, 2013 at 03:26:24PM -0400, Vlad Yasevich wrote:
> On 08/15/2013 03:20 PM, Michael S. Tsirkin wrote:
> >On Thu, Aug 15, 2013 at 10:09:45PM +0300, Michael S. Tsirkin wrote:
> >>On Thu, Aug 15, 2013 at 02:45:30PM -0400, Vlad Yasevich wrote:
> >>>On 08/15/2013 02:24 PM, Michael S. Tsirkin wrote:
> >>>>On Thu, Aug 15, 2013 at 01:02:52PM -0400, Vlad Yasevich wrote:
> >>>>>The features of the macvlan are based on the features of lower
> >>>>>device and thus can have checksum featurs other then IFF_F_HW_CSUM
> >>>>
> >>>>s/featurs/features/
> >>>>
> >>>>:set spell spelllang=en_us
> >>>>
> >>>>or whatever's the equivalent in your editor of choice.
> >>>>
> >>>>>set.  However, TUN_OFFLOAD mask only includes IFF_F_HW_CSUM.  Thus
> >>>>>when performing gso segmentation during macvtap_forward(),
> >>>>>it is possbile to end up with skbs that have ip_summed set
> >>>>>to CHECKSUM_PARTIAL.  This is incorrect when the user
> >>>>>turns off checksum offloading.
> >>>>>
> >>>>>Include all possible checksum offload values so that
> >>>>>we'll properly mask them off when performing GSO.
> >>>>>
> >>>>>Signed-off-by: Vlad Yasevich <vyasevic@...hat.com>
> >>>>>---
> >>>>>  drivers/net/macvtap.c | 2 +-
> >>>>>  1 file changed, 1 insertion(+), 1 deletion(-)
> >>>>>
> >>>>>diff --git a/drivers/net/macvtap.c b/drivers/net/macvtap.c
> >>>>>index b51db2a..8121358 100644
> >>>>>--- a/drivers/net/macvtap.c
> >>>>>+++ b/drivers/net/macvtap.c
> >>>>>@@ -65,7 +65,7 @@ static struct cdev macvtap_cdev;
> >>>>>
> >>>>>  static const struct proto_ops macvtap_socket_ops;
> >>>>>
> >>>>>-#define TUN_OFFLOADS (NETIF_F_HW_CSUM | NETIF_F_TSO_ECN | NETIF_F_TSO | \
> >>>>>+#define TUN_OFFLOADS (NETIF_F_ALL_CSUM | NETIF_F_TSO_ECN | NETIF_F_TSO | \
> >>>>>  		      NETIF_F_TSO6 | NETIF_F_UFO)
> >>>>>  #define RX_OFFLOADS (NETIF_F_GRO | NETIF_F_LRO)
> >>>>>  /*
> >>>>
> >>>>Okay so you are talking about hardware that sets some other
> >>>>checksum bit besides HW_CSUM, e.g. IP_CSUM, so
> >>>>
> >>>>         vlan->tap_features = vlan->dev->features &
> >>>>                             (feature_mask | ~TUN_OFFLOADS);
> >>>>
> >>>>will not clear IP_CSUM even if feature_mask is 0.
> >>>>
> >>>>Maybe mention this in the changelog, in case user
> >>>>will wonder whether his hardware is affected.
> >>>>
> >>>>So I agree, that's a bug, but if you make this change the reverse will hold
> >>>>(on this hardware):
> >>>>if user sets TUN_F_CSUM, we won't set NETIF_F_IP_CSUM
> >>>>so checksum offloading won't work.
> >>>>
> >>>>So I think you want s/NETIF_F_HW_CSUM/NETIF_F_ALL_CSUM/ everywhere
> >>>>and not just in this place.
> >>>
> >>>Yes.  I thought about that, but forgot to do in this patch set.
> >>
> >>In fact I wonder why do we do it like this at all.
> >>tap_features have nothing to do with the device,
> >>they only have to do with tap.
> >>For example, you might have a dumb device with no
> >>checksum support but userspace said it does not need a checksum,
> >>we can have NETIF_F_HW_CSUM there anyway.
> >>
> >>So why don't we just
> >>         vlan->tap_features = feature_mask;
> >>?
> >>In fact, this is exactly set_features, so why don't
> >>we just use set_features everywhere?
> >>Then this patch won't be needed at all.
> >>
> >>Kind of like this - compile-tested only - do you have a setup with
> >>IP_CSUM which you can use to test?
> >
> >Ugh this does not work even on a simple setup, sorry about the noise.
> >Maybe like this? Does this work for you?
> >
> 
> No, this is really no different then what I had to start with.
> 
> >-->
> >
> >macvtap: fix up tap features
> >
> >There's no apparent need to have tap features
> >masked according to the lowerdev config:
> >we only use them in software so hardware configuration
> >does not matter.
> >
> >This patch fixes bugs for some unusual hardware configurations,
> >for example, we only masked HW_CSUM so for hardware with
> >e.g. IP_CSUM the checksum bit remained set, which would cause
> >packets with CHECKSUM_PARTIAL to be sent to userspace
> >even when offloads are disabled.
> >
> >Signed-off-by: Michael S. Tsirkin <mst@...hat.com>
> >
> >---
> >
> >diff --git a/drivers/net/macvtap.c b/drivers/net/macvtap.c
> >index a98ed9f..3c18f12 100644
> >--- a/drivers/net/macvtap.c
> >+++ b/drivers/net/macvtap.c
> >@@ -1058,8 +1058,7 @@ static int set_offload(struct macvtap_queue *q, unsigned long arg)
> >  	/* tap_features are the same as features on tun/tap and
> >  	 * reflect user expectations.
> >  	 */
> >-	vlan->tap_features = vlan->dev->features &
> >-			    (feature_mask | ~TUN_OFFLOADS);
> >+	vlan->tap_features = feature_mask;
> 
> This wouldn't work when tap_features = 0, since the skb features in
> forward would evaluate to 0.  This means that netif_needs_gso() will
> return false and a GSO packet will be queued to the socket without
> going though segmentation.
> 
> -vlad
> 

Hmm won't same thing happen if features == 0 on lowerdev?


> >  	vlan->set_features = features;
> >  	netdev_update_features(vlan->dev);
> >
> >
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