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[Qemu-devel] qemu -netdev tun/tap support can't handle macvtap type devi
From: |
Peter Maydell |
Subject: |
[Qemu-devel] qemu -netdev tun/tap support can't handle macvtap type devices |
Date: |
Wed, 7 Dec 2016 12:01:14 +0000 |
Hi; this is a bug report I'm forwarding for somebody else.
The problem is that QEMU's tun/tap support in net/tap-linux.c only
supports the legacy /dev/net/tun interface (the filename is hardcoded).
If you created the tap device via macvtap, then this is the wrong
filename, but there's no code in QEMU to get the correct /dev/tapN
filename from the user-provided ifname=whatever option.
Talking on IRC, the answer suggested was that we ought to
do a SIOCGIFINDEX ioctl passing in the user-specified ifname
string, which then gives you the index N to construct the
/dev/tapN filename. There is probably complexity in working out
whether we should do that or use the legacy interface (or try both
always?) This is probably overall not a very large amount of code,
though.
The usual suggested workaround is to use the -netdev fd option, like
fd=3 3<>/dev/tap$(< /sys/class/net/tap0/ifindex)
(which gets the shell to open the right /dev/tap device).
Unfortunately this isn't compatible with multi-queue support
because netdev complains
"ifname=, script=, downscript=, vnet_hdr=, helper=, queues=, fds=,
and vhostfds= are invalid with fd="
so you can't pass options like "queues=4"...
Detailed repro case below if required (uses aarch64 but none
of this is arch-specific as far as I know).
------------<Begin Reproducer>--------------
To reproduce, here are the steps I followed.
Create a tap device on the host
===============================
# ip link add link eth0 name tap0 type macvtap mode bridge
To create the tap device the host kernel needs CONFIG_MACVTAP enabled.
You can check that the device is created by running
$ ip a
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state
UNKNOWN group default qlen 1
link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
inet 127.0.0.1/8 scope host lo
valid_lft forever preferred_lft forever
inet6 ::1/128 scope host
valid_lft forever preferred_lft forever
2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc mq state
UP group default qlen 1000
link/ether 02:a1:a2:a3:a4:a5 brd ff:ff:ff:ff:ff:ff
inet 10.1.194.30/23 brd 10.1.195.255 scope global eth0
valid_lft forever preferred_lft forever
inet6 fe80::a1:a2ff:fea3:a4a5/64 scope link
valid_lft forever preferred_lft forever
[...]
8: address@hidden: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc
pfifo_fast state UP group default qlen 500
link/ether 5e:fc:d2:28:f2:f9 brd ff:ff:ff:ff:ff:ff
inet6 fe80::5cfc:d2ff:fe28:f2f9/64 scope link
valid_lft forever preferred_lft forever
Creating a tap device creates a character device. The character device
created is "/dev/tap" followed by the interface index (8 in our example above).
The interface index can also be discovered by reading a node in sysfs.
$ cat /sys/class/net/tap0/ifindex
8
Modify tap device permissions
=============================
By default, the device node is created with permissions root:root.
$ ls -l /dev/tap8
crw------- 1 root root 242, 1 Nov 30 11:57 /dev/tap8
Update the permissions so that qemu-system-aarch64 can access the node.
# chgrp kvm /dev/tap8
# chmod g+rw /dev/tap8
$ ls -l /dev/tap8
crw-rw---- 1 root kvm 242, 1 Nov 30 11:57 /dev/tap8
Run qemu
========
I am using a locally built qemu (tag: v2.8.0-rc2). When attempting to
start a guest I get the following complaint -
$ qemu-system-aarch64 -M virt -m 4096 -cpu host -smp 4 -enable-kvm
-nographic \
-pflash flash0.img -pflash flash1.img \
-drive if=none,file=hda.qcow2,id=hd0" \
-device virtio-blk-device,drive=hd0 \
-netdev tap,id=eth0,script=,ifname=tap0,queues=4 \
-device
virtio-net-pci,netdev=eth0,mq=on,vectors=12,mac=22:22:3a:57:07:ed
qemu-system-aarch64: -netdev
tap,id=eth0,script=,ifname=tap0,queues=4: could not open /dev/net/tun:
No such file or directory
------------<End of Reproducer>--------------
thanks
-- PMM
- [Qemu-devel] qemu -netdev tun/tap support can't handle macvtap type devices,
Peter Maydell <=
- Re: [Qemu-devel] qemu -netdev tun/tap support can't handle macvtap type devices, Daniel P. Berrange, 2016/12/07
- Re: [Qemu-devel] qemu -netdev tun/tap support can't handle macvtap type devices, Peter Maydell, 2016/12/07
- Re: [Qemu-devel] qemu -netdev tun/tap support can't handle macvtap type devices, Daniel P. Berrange, 2016/12/07
- Re: [Qemu-devel] qemu -netdev tun/tap support can't handle macvtap type devices, Peter Maydell, 2016/12/07
- Re: [Qemu-devel] qemu -netdev tun/tap support can't handle macvtap type devices, Daniel P. Berrange, 2016/12/07
- Re: [Qemu-devel] qemu -netdev tun/tap support can't handle macvtap type devices, Peter Maydell, 2016/12/07
- Re: [Qemu-devel] qemu -netdev tun/tap support can't handle macvtap type devices, Jason Wang, 2016/12/07
- Re: [Qemu-devel] qemu -netdev tun/tap support can't handle macvtap type devices, Peter Maydell, 2016/12/08
- Re: [Qemu-devel] qemu -netdev tun/tap support can't handle macvtap type devices, Jason Wang, 2016/12/07