Issue

On the Host, even using -opost, which tells to your serial port to ignore any enabled specific post-processing option:

[root@host ~]# stty -F /dev/ttyS4 -opost

If a qemu Guest inside the Host tries to transmit data using a virtual serial port mapped to the physical serial ttyS4, the data transmitted will be affected by post-processing options.

For example, in a given qemu Guest with /dev/ttyS0 mapped to Host /dev/ttyS4, writing the following characters on guest serial device /dev/ttyS0:

 0x41
 0x0A
 0x42

The data captured from Host serial device /dev/ttyS4 will be:

 0x41
 0x0D
 0x0A
 0x42

In this case, the data is affected by onlcr post-processing option, translating newline (0x0A) to carriage return-newline (0x0D).