If netdev_add tap,id=net0,...,vhost=on failed in net_init_tap_one(),
the followed up device_add virtio-net-pci,netdev=net0 will fail
too, prints:

   TUNSETOFFLOAD ioctl() failed: Bad file descriptor TUNSETOFFLOAD
   ioctl() failed: Bad file descriptor

The reason is that the fd of tap is closed when error occured after
calling net_init_tap_one().

The fd should be closed when calling net_init_tap_one failed:
   - if tap_set_sndbuf() failed
   - if tap_set_sndbuf() succeeded but vhost failed to open or
     initialize with vhostforce flag on
   - with wrong vhost command line parameter
The fd should not be closed just because vhost failed to open or
initialize but without vhostforce flag. So the followed up
device_add can fall back to userspace virtio successfully.

Suggested-by: Michael S. Tsirkin <mst@redhat.com>
Suggested-by: Igor Mammedov <imammedo@redhat.com>
Suggested-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: Jay Zhou <jianjay.zhou@huawei.com>
---
 include/net/vhost_net.h |  3 +++
 net/tap.c               | 22 +++++++++++++++++-----
 2 files changed, 20 insertions(+), 5 deletions(-)

diff --git a/include/net/vhost_net.h b/include/net/vhost_net.h
index afc1499..77e4739 100644
--- a/include/net/vhost_net.h
+++ b/include/net/vhost_net.h
@@ -4,6 +4,9 @@
 #include "net/net.h"
 #include "hw/virtio/vhost-backend.h"
 
+#define VHOST_NET_INIT_FAILED \
+    "vhost-net requested but could not be initialized"
+
 struct vhost_net;
 typedef struct vhost_net VHostNetState;
 
diff --git a/net/tap.c b/net/tap.c
index 979e622..2b3a36f 100644
--- a/net/tap.c
+++ b/net/tap.c
@@ -686,14 +686,23 @@ static void net_init_tap_one(const NetdevTapOptions *tap, NetClientState *peer,
         if (vhostfdname) {
             vhostfd = monitor_fd_param(cur_mon, vhostfdname, &err);
             if (vhostfd == -1) {
-                error_propagate(errp, err);
+                if (tap->has_vhostforce && tap->vhostforce) {
+                    error_propagate(errp, err);
+                } else {
+                    warn_report_err(err);
+                }
                 return;
             }
         } else {
             vhostfd = open("/dev/vhost-net", O_RDWR);
             if (vhostfd < 0) {
-                error_setg_errno(errp, errno,
-                                 "tap: open vhost char device failed");
+                if (tap->has_vhostforce && tap->vhostforce) {
+                    error_setg_errno(errp, errno,
+                                     "tap: open vhost char device failed");
+                } else {
+                    warn_report("tap: open vhost char device failed: %s",
+                                strerror(errno));
+                }
                 return;
             }
             fcntl(vhostfd, F_SETFL, O_NONBLOCK);
@@ -702,8 +711,11 @@ static void net_init_tap_one(const NetdevTapOptions *tap, NetClientState *peer,
 
         s->vhost_net = vhost_net_init(&options);
         if (!s->vhost_net) {
-            error_setg(errp,
-                       "vhost-net requested but could not be initialized");
+            if (tap->has_vhostforce && tap->vhostforce) {
+                error_setg(errp, VHOST_NET_INIT_FAILED);
+            } else {
+                warn_report(VHOST_NET_INIT_FAILED);
+            }
             return;
         }
     } else if (vhostfdname) {
-- 
1.8.3.1

On 2018年03月02日 17:04, Jay Zhou wrote:
> If netdev_add tap,id=net0,...,vhost=on failed in net_init_tap_one(),
> the followed up device_add virtio-net-pci,netdev=net0 will fail
> too, prints:
>
>     TUNSETOFFLOAD ioctl() failed: Bad file descriptor TUNSETOFFLOAD
>     ioctl() failed: Bad file descriptor
>
> The reason is that the fd of tap is closed when error occured after
> calling net_init_tap_one().
>
> The fd should be closed when calling net_init_tap_one failed:
>     - if tap_set_sndbuf() failed
>     - if tap_set_sndbuf() succeeded but vhost failed to open or
>       initialize with vhostforce flag on
>     - with wrong vhost command line parameter
> The fd should not be closed just because vhost failed to open or
> initialize but without vhostforce flag. So the followed up
> device_add can fall back to userspace virtio successfully.
>
> Suggested-by: Michael S. Tsirkin <mst@redhat.com>
> Suggested-by: Igor Mammedov <imammedo@redhat.com>
> Suggested-by: Jason Wang <jasowang@redhat.com>
> Signed-off-by: Jay Zhou <jianjay.zhou@huawei.com>
> ---
>   include/net/vhost_net.h |  3 +++
>   net/tap.c               | 22 +++++++++++++++++-----
>   2 files changed, 20 insertions(+), 5 deletions(-)
>
> diff --git a/include/net/vhost_net.h b/include/net/vhost_net.h
> index afc1499..77e4739 100644
> --- a/include/net/vhost_net.h
> +++ b/include/net/vhost_net.h
> @@ -4,6 +4,9 @@
>   #include "net/net.h"
>   #include "hw/virtio/vhost-backend.h"
>   
> +#define VHOST_NET_INIT_FAILED \
> +    "vhost-net requested but could not be initialized"
> +
>   struct vhost_net;
>   typedef struct vhost_net VHostNetState;
>   
> diff --git a/net/tap.c b/net/tap.c
> index 979e622..2b3a36f 100644
> --- a/net/tap.c
> +++ b/net/tap.c
> @@ -686,14 +686,23 @@ static void net_init_tap_one(const NetdevTapOptions *tap, NetClientState *peer,
>           if (vhostfdname) {
>               vhostfd = monitor_fd_param(cur_mon, vhostfdname, &err);
>               if (vhostfd == -1) {
> -                error_propagate(errp, err);
> +                if (tap->has_vhostforce && tap->vhostforce) {
> +                    error_propagate(errp, err);
> +                } else {
> +                    warn_report_err(err);
> +                }
>                   return;
>               }
>           } else {
>               vhostfd = open("/dev/vhost-net", O_RDWR);
>               if (vhostfd < 0) {
> -                error_setg_errno(errp, errno,
> -                                 "tap: open vhost char device failed");
> +                if (tap->has_vhostforce && tap->vhostforce) {
> +                    error_setg_errno(errp, errno,
> +                                     "tap: open vhost char device failed");
> +                } else {
> +                    warn_report("tap: open vhost char device failed: %s",
> +                                strerror(errno));
> +                }
>                   return;
>               }
>               fcntl(vhostfd, F_SETFL, O_NONBLOCK);
> @@ -702,8 +711,11 @@ static void net_init_tap_one(const NetdevTapOptions *tap, NetClientState *peer,
>   
>           s->vhost_net = vhost_net_init(&options);
>           if (!s->vhost_net) {
> -            error_setg(errp,
> -                       "vhost-net requested but could not be initialized");
> +            if (tap->has_vhostforce && tap->vhostforce) {
> +                error_setg(errp, VHOST_NET_INIT_FAILED);
> +            } else {
> +                warn_report(VHOST_NET_INIT_FAILED);
> +            }
>               return;
>           }
>       } else if (vhostfdname) {

Applied, thanks.