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As Eduardo sayed in bug 664635:

Just don't stop reading data, this is asking for trouble.

We could try to fix that by implementing flow control everythere, so the guest gets signaled via serial port status registers that the pipe is full and stops writing more.  This depends on pimping up qemu chardevs, which in turn depends on glib main loop, which pretty much implies it is RHEL-7 material.

I suspect that still wouldn't help much for linux kernel messages as the serial console writes are done syncronously.  You can see that even on bare metal, configure a serial line with 9600 baud and watch how this slows down the kernel boot.  In the end qemu wouldn't block, but the linux kernel would block instead.