[Qemu-devel] qemu guest stalls with lots of PTY writes

[Qemu-devel] qemu guest stalls with lots of PTY writes

[Stephen Su]

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qemu experts, please help.

This is seen with the latest qemu 1.5 (commit 62c339c5272ce8fbe8ca52695cee8ff40da7872e), guest VM stalls intermittently. 


Setup:2) start a linux guest with a PTY device, for example


    <serial type='pty'>
      <source path='/dev/pts/4'/>
      <target port='0'/>
      <alias name='serial0'/>
    </serial>
    <console type='pty' tty='/dev/pts/4'>
      <source path='/dev/pts/4'/>
      <target type='serial' port='0'/>
      <alias name='serial0'/>
    </console>

3) Bind pty device to a slow tty device
socat -d /dev/pts/4 /dev/ttyS0,raw,b9600,echo=0

4) on serial console, writes lots of message, for example
repeat 10000 echo "`date`: this is a long long long long long long long test message"

5) on the host, 'strace -f -p' kvm device, noticed that trace stalls once a while. In addition, if you login via ssh to the guest, note that the login shell stalls intermittently.

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Re: [Qemu-devel] qemu guest stalls with lots of PTY writes

[Andrey Korolyov]

Hello,

please take a look on
http://lists.nongnu.org/archive/html/qemu-devel/2014-09/msg00600.html,
I had reported the simular issue for recent emulator versions. It
would be good if this particular problem can achieve more votes,
because I have not enough knowledge to fix it by myself.