SerRecv thread using 100% cpu on host

[dr-grog]

The SerRecv thread is using 100% cpu on the host, while the guest is idle (<1% cpu). The guest only uses the serial port to transmit & receive about 100 bytes of data every 2 minutes. The high cpu on the host starts as soon as the VBoxHeadless process is started, well before the guest even opens the port or receives any data. The guest is still able to read and write to the port ok.

top - 14:50:52 up 15:21,  3 users,  load average: 1.15, 0.48, 0.49
Tasks: 188 total,   1 running, 187 sleeping,   0 stopped,   0 zombie
%Cpu(s):  9.7 us, 42.5 sy,  0.0 ni, 47.7 id,  0.0 wa,  0.0 hi,  0.2 si,  0.0 st
KiB Mem:   4054072 total,  3511776 used,   542296 free,   278388 buffers
KiB Swap:  4194300 total,        0 used,  4194300 free.  1994828 cached Mem

  PID USER      PR  NI    VIRT    RES    SHR S  %CPU %MEM     TIME+ COMMAND
15684 zzzzzzzz  20   0  482552 415340 377300 S 101.1 10.2   1:55.55 VBoxHeadless

  PID USER      PR  NI    VIRT    RES    SHR S %CPU %MEM     TIME+ COMMAND
15706 zzzzzzzz  20   0  488696 421316 383444 R 99.9 10.4   2:13.19 SerRecv

The uart settings on the host are: UART 1: I/O base: 0x03f8, IRQ: 4, attached to device '/dev/ttyS0'

The high cpu can be prevented by setting --uart1 off or --uartmode1 disconnected, however the serial port cannot be used so it's not a solution.

Host and guest are both Ubuntu 14.04.3 LTS. Problem was originally reported on the forums - ​https://forums.virtualbox.org/viewtopic.php?f=7&t=74078

[dr-grog]

One or more of the following seems to have fixed the issue: upgraded to 5.0.8; rebuilt vbox modules on the host; updated guest kernel to 3.13.0-66 & install 5.0.8 guest-additions;

Not sure if this is related in any way to #14659 which is also a serial port issue in the 5.0.8 changelog. It's certainly not the same as I was still able to use the port, but at the expense of cpu hogging. Don't see how how r58015 is related to SerRecv, but I'm no expert.

(side-note: the new /sbin/vboxconfig didn't seem to work for me for rebuilding the modules, and in this case I still had to use /etc/init.d/vboxdrv setup)

[Frank Mehnert]

Could be very well the 5.0.8 fix. Let's close this ticket for now. Please reopen if you experience the same issue with 5.0.8 or later again. Thank you!