Re: [PATCH net] vhost_net: remove VHOST_NET_F_VIRTIO_NET_HDR support

From: Jason Wang
Date: Sun Jun 10 2018 - 22:29:58 EST

On 2018å06æ11æ 10:12, Michael S. Tsirkin wrote:
On Fri, Jun 08, 2018 at 01:07:09PM +0800, Jason Wang wrote:

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
Yes but that's the admittedly ugly API that we have now.
userspace is supposed to know VRITIO_F_ANY_LAYOUT does
not make sense for vhost.


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?
That doesn't mean much.

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.
Sounds good, so if you like, reserve a bit for
VHOST_NET_F_VIRTIO_NET_HDR in the new ioctl mask and
do not enable it there.

Ok, and maybe VHOST_F_LOG_ALL.

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.

Well ANY_LAYOUT just happens to be same as VHOST_NET_F_VIRTIO_NET_HDR
and that has been set since forever.

So do you still want this patch? If not we need to document that ANY_LAYOUT could not be passed through SET_FEATURES somewhere.


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 {
@@ -1255,15 +1254,11 @@ static int vhost_net_set_features(struct vhost_net *n, u64 features)
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;
if ((features & (1 << VHOST_F_LOG_ALL)) &&