Red Hat Bugzilla – Bug 987278
fail to transfer any data between host and guest with virtio serial through UDP option
Last modified: 2014-12-15 01:52:25 EST
host info: 3.10.0-0.rc7.64.el7.x86_64 qemu-kvm-1.5.1-2.el7.x86_64 guest info: 3.10.0-0.rc7.64.el7.x86_64
udp chardev works for me. Terminal #1: nc -4 -u -l 12345 Terminal #2: qemu-kvm -chardev udp,id=test,port=12345,localport=54321,ipv4 \ -device isa-debugcon,iobase=0x402,chardev=test seabios log messages show up in Terminal #1 just fine. Now and then there is a piece missing. But note that unlike TCP the UDP protocol doesn't make any guaranties whatsoever. In case buffers are full packets are simply dropped and lost. So this is normal behavior. Packets can also be reordered. Note that both qemu+netcat are forced into ipv4 mode to make sure they speak the same protocol. Also note that I'm explicitly setting localport to avoid a random port being picked. This allows to keep netcat in terminal #1 running while starting qemu multiple times even though netcat will only accept packets from a single peer address. Final note: The UDP protocol is stateless. There is no "connected" and "disconnected" state. Consequently the UDP chardev backend doesn't send open/close chardev events. Which virtio-serial might depend on to work correctly. Amit? Bottom line: Testing the udp chardev with anything which requires a reliable stream is bogus in the first place. The only remotely useful use case for the udp chardev backend I can think of is to send guest output to a remote syslog server (which traditionally listens on udp port 514).
(In reply to Gerd Hoffmann from comment #3) > Bottom line: Testing the udp chardev with anything which requires a > reliable stream is bogus in the first place. The only remotely useful use > case for the udp chardev backend I can think of is to send guest output to a > remote syslog server (which traditionally listens on udp port 514). I think that the right test is to validate that data is received. If some/many of the packets are lost, this is OK, as long as many packets are also received.
(In reply to Gerd Hoffmann from comment #3) > Final note: The UDP protocol is stateless. There is no "connected" and > "disconnected" state. Consequently the UDP chardev backend doesn't send > open/close chardev events. Which virtio-serial might depend on to work > correctly. Amit? Yep, good point.