semantic: 0.254
other: 0.248
device: 0.087
network: 0.072
graphic: 0.055
performance: 0.046
debug: 0.037
vnc: 0.037
files: 0.036
permissions: 0.036
socket: 0.035
PID: 0.028
boot: 0.018
KVM: 0.010
debug: 0.222
files: 0.157
device: 0.114
PID: 0.091
other: 0.090
network: 0.075
socket: 0.056
semantic: 0.044
performance: 0.033
boot: 0.028
graphic: 0.026
permissions: 0.021
KVM: 0.021
vnc: 0.021

qemu: invalid serial port configuration

The tty_serial_init() function sets the port c_oflags as follows:
tty.c_oflag |= OPOST not clearing ONLCR, ONLRET and others.
The result is that the postprocess output is enabled and host translates 0xa (LF) to 0xd 0xa (CR LF) which breaks the binary transmissions on serial port even if you set the port to raw mode (no matters if on host and/or guest).
The issue has been reported 11 years ago on qemu-devel mailing list:
https://lists.nongnu.org/archive/html/qemu-devel/2006-06/msg00196.html
There was also a FreeBSD patch including the fix:
https://lists.freebsd.org/pipermail/freebsd-ports/2006-October/036390.html

I think the correct port configuration is:
tty.c_iflag &= ~(IGNBRK|BRKINT|PARMRK|ISTRIP|INLCR|IGNCR|ICRNL|IXON|IMAXBEL);
tty.c_oflag &= ~OPOST;

In such case the host will perform no output processing and will pass the data as is.
And the guest will be able to configure input/output processing exactly as it wants.

I believe the following bug is related: https://bugs.launchpad.net/qemu/+bug/1407813

Patch has now been committed here:
https://git.qemu.org/?p=qemu.git;a=commitdiff;h=12fb0ac0575df83cec72ec