Re: [PATCH net] vhost_net: remove VHOST_NET_F_VIRTIO_NET_HDR support

[Jason Wang]

On 2018å06æ08æ 12:46, Michael S. Tsirkin wrote:
> On Fri, Jun 08, 2018 at 11:50:42AM +0800, Jason Wang wrote:
> > This feature bit is duplicated with VIRTIO_F_ANY_LAYOUT, this means if
> > a userpsace want to enable VRITIO_F_ANY_LAYOUT,
> > VHOST_NET_F_VIRTIO_NET_HDR will be implied too. This is wrong and will
> > break networking.
> What breaks networking exactly? VHOST_NET supported ANY_LAYOUT
> from day one. For this reason it does not need to know about
> VRITIO_F_ANY_LAYOUT and we reused the bit for other purposes.

It's the knowledge of vhost_net code it self but not userspace. For 
userspace, it should depends on the value of returned by 
VHOST_GET_FEATURES. So when userspace can set_features with ANY_LAYOUT, 
vhost may think it wants VHOST_NET_F_VIRTIO_NET_HDR.

> > Fixing this by safely removing
> > VHOST_NET_F_VIRTIO_NET_HDR support. There should be very few or even
> > no userspace can use this.
> Quite possibly, but it is hard to be sure. It seems safer to
> maintain it unless there's an actual reason something's broken.

I think not since the feature is negotiated not mandatory?

> > Further cleanups could be done for
> > -net-next for safety.
> >
> > In the future, we need a vhost dedicated feature set/get ioctl()
> > instead of reusing virtio ones.
> Not just in the future, we might want to switch iommu
> to a sane structure without the 64 bit padding bug
> right now.

Yes, I hit this bug when introducing V2 of msg IOTLB message.

> > Fixes: 4e9fa50c6ccbe ("vhost: move features to core")
> This tag makes no sense here IMHO. Looks like people are using some tool
> that just looks at the earliest version where patch won't apply. The
> commit in question just moved some code around.

Looks not, before this commit, vhost_net won't return ANY_LAYOUT.

Thanks

> > Signed-off-by: Jason Wang <jasowang@xxxxxxxxxx>
> > ---
> >   drivers/vhost/net.c | 15 +++++----------
> >   1 file changed, 5 insertions(+), 10 deletions(-)
> >
> > diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
> > index 986058a..83eef52 100644
> > --- a/drivers/vhost/net.c
> > +++ b/drivers/vhost/net.c
> > @@ -69,7 +69,6 @@ MODULE_PARM_DESC(experimental_zcopytx, "Enable Zero Copy TX;"
> >   
> >   enum {
> >   	VHOST_NET_FEATURES = VHOST_FEATURES |
> > -			 (1ULL << VHOST_NET_F_VIRTIO_NET_HDR) |
> >   			 (1ULL << VIRTIO_NET_F_MRG_RXBUF) |
> >   			 (1ULL << VIRTIO_F_IOMMU_PLATFORM)
> >   };
> > @@ -1255,15 +1254,11 @@ static int vhost_net_set_features(struct vhost_net *n, u64 features)
> >   			       (1ULL << VIRTIO_F_VERSION_1))) ?
> >   			sizeof(struct virtio_net_hdr_mrg_rxbuf) :
> >   			sizeof(struct virtio_net_hdr);
> > -	if (features & (1 << VHOST_NET_F_VIRTIO_NET_HDR)) {
> > -		/* vhost provides vnet_hdr */
> > -		vhost_hlen = hdr_len;
> > -		sock_hlen = 0;
> > -	} else {
> > -		/* socket provides vnet_hdr */
> > -		vhost_hlen = 0;
> > -		sock_hlen = hdr_len;
> > -	}
> > +
> > +        /* socket provides vnet_hdr */
> > +	vhost_hlen = 0;
> > +	sock_hlen = hdr_len;
> > +
> >   	mutex_lock(&n->dev.mutex);
> >   	if ((features & (1 << VHOST_F_LOG_ALL)) &&
> >   	    !vhost_log_access_ok(&n->dev))
> > --
> > 2.7.4