tap: introduce a helper to get the name of an interface

This patch introduces a helper tap_get_ifname() to get the device name of tap
device. This is needed when ifname is unspecified in the command line and qemu
were asked to create tap device by itself. In this situation, the name were
allocated by kernel, so if multiqueue is asked, we need to fetch its name after
creating the first queue.

Only linux has this support since it's the only platform that supports
multiqueue tap.

Signed-off-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>

include/net/tap.h

@@ -37,6 +37,7 @@ void tap_set_offload(NetClientState *nc, int csum, int tso4, int tso6, int ecn,
 void tap_set_vnet_hdr_len(NetClientState *nc, int len);
 int tap_enable(NetClientState *nc);
 int tap_disable(NetClientState *nc);
+int tap_get_ifname(NetClientState *nc, char *ifname);
 
 int tap_get_fd(NetClientState *nc);
 

net/tap-aix.c

@@ -69,3 +69,9 @@ int tap_fd_disable(int fd)
 {
     return -1;
 }
+
+int tap_fd_get_ifname(int fd, char *ifname)
+{
+    return -1;
+}
+

net/tap-bsd.c

@@ -156,3 +156,7 @@ int tap_fd_disable(int fd)
     return -1;
 }
 
+int tap_fd_get_ifname(int fd, char *ifname)
+{
+    return -1;
+}

net/tap-haiku.c

@@ -70,3 +70,7 @@ int tap_fd_disable(int fd)
     return -1;
 }
 
+int tap_fd_get_ifname(int fd, char *ifname)
+{
+    return -1;
+}

net/tap-linux.c

@@ -261,3 +261,16 @@ int tap_fd_disable(int fd)
     return ret;
 }
 
+int tap_fd_get_ifname(int fd, char *ifname)
+{
+    struct ifreq ifr;
+
+    if (ioctl(fd, TUNGETIFF, &ifr) != 0) {
+        error_report("TUNGETIFF ioctl() failed: %s",
+                     strerror(errno));
+        return -1;
+    }
+
+    pstrcpy(ifname, sizeof(ifr.ifr_name), ifr.ifr_name);
+    return 0;
+}

net/tap-solaris.c

@@ -236,3 +236,7 @@ int tap_fd_disable(int fd)
     return -1;
 }
 
+int tap_fd_get_ifname(int fd, char *ifname)
+{
+    return -1;
+}

net/tap_int.h

@@ -44,5 +44,6 @@ void tap_fd_set_offload(int fd, int csum, int tso4, int tso6, int ecn, int ufo);
 void tap_fd_set_vnet_hdr_len(int fd, int len);
 int tap_fd_enable(int fd);
 int tap_fd_disable(int fd);
+int tap_fd_get_ifname(int fd, char *ifname);
 
 #endif /* QEMU_TAP_H */