Red Hat Bugzilla – Bug 617220
KVM guest hangs when writing to unopen virtio-console port.
Last modified: 2014-11-30 16:13:47 EST
Description of problem:
KVM guest hangs after opening the virtio-console port, when data already in virtioconsole buffer.
Version-Release number of selected component (if applicable):
Steps to Reproduce:
1. Start qemu-kvm with commandline
qemu-kvm -device virtio-serial -drive file=/data/Virtual/fedora-Vconsole.img,if=virtio,boot=on,serial=s,cache=writeback -m 512 -chardev socket,path=/tmp/aaa,id=aaa,server,nowait -device virtconsole,chardev=aaa,name=org.fedoraproject.console.aaa -enable-kvm -monitor stdio -chardev socket,path=/tmp/b,id=b,server,nowait -device virtconsole,chardev=b,name=org.fedoraproject.console.b
2. send 10KB to port /tmp/b
3. in guest open (/dev/hvc1)
Guest hangs in infinite loops (cpu usage 100% never end)
On guest side open console /dev/hvc1 and recieve data.
I test same stuff using virtserialport with positive results.
1) Start guest
2) First send data from host (socket /tmp/data) to serial port to guest (/dev/vport0p1).
3) after data are sent open port with 'cat /dev/vport0p1'
cat listens on port, old data are truncated and new are correctly received.
Works for me with Linus' git kernel in the guest. Most likely looks like a guest kernel issue. I'll update when I have more data or a fix.
I believe this is the same issue as I discussed with Amit on irc this is a feature ^W bug on how the virtio_console driver in the guest handles writes when no one is listening on the host side.
The problem is this "beauty" in virtio_console.c: send_buf()
* Wait till the host acknowledges it pushed out the data we
* sent. This is done for ports in blocking mode or for data
* from the hvc_console; the tty operations are performed with
* spinlocks held so we can't sleep here.
while (!virtqueue_get_buf(out_vq, &len))
I see a number of possible (partial) solutions here:
1) the code says it is using cpu_relux rather then sleep, because the tty functions are called with a spinlock held. but fops-write is not called with any spinlock held, I believe. How about a parameter to send_buf, called "may_sleep" and then use sleep rather then relax if may_sleep is true?
2) The waiting is done for: "This is done for ports in blocking mode or for data from the hvc_console". I wonder why the waiting is done in blocking mode too, I guess this is some sort of workaround for the missing waitqueue wakeups, see bug 643750, with those waitqueue wakeups added I would think / expect the waiting for the host acknowledge is only needed for tty usage, and that we could skip the wait entirely (making 1 mute) when called from fops_write ?
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