QEMU doesn't create a second serial port (Ubuntu x86-64 guest and host)

Question

I run my QEMU version 3.0.0 by:
x86_64-softmmu/qemu-system-x86_64 -m 2560 -hda img.qcow2 -serial pty -serial pty

My host is an Ubuntu Desktop 16.04 and my guest is a fresh (i.e. default configuration) Ubuntu Server 18.04. Both are x86-64.

Upon starting, QEMU immediately prints:

qemu-system-x86_64: -serial pty: char device redirected to /dev/pts/18 (label serial0)
qemu-system-x86_64: -serial pty: char device redirected to /dev/pts/19 (label serial1)

Also, the output of info qtree (in QEMU's monitor) is:

[...]
          dev: isa-serial, id ""
            index = 1 (0x1)
            iobase = 760 (0x2f8)
            irq = 3 (0x3)
            chardev = "serial1"
            wakeup = 0 (0x0)
            isa irq 3
          dev: isa-serial, id ""
            index = 0 (0x0)
            iobase = 1016 (0x3f8)
            irq = 4 (0x4)
            chardev = "serial0"
            wakeup = 0 (0x0)
            isa irq 4
[...]

So it seems that serial0 and serial1 should be quite the same.
According to this site, /dev/ttyS1 should be used for the serial port whose iobase is 0x2f8, so from the output above I deduced that serial1 should be used through /dev/ttyS1.

I can work without any issue with /dev/ttyS0 in the guest and /dev/pts/18 in the host. However, I didn't manage to work with /dev/ttyS1 in the guest, and it seems to me that serial1 doesn't exist at all in the guest.

Inside the guest, the output of dmesg | grep ttyS (which should show the existing serial ports, according to this answer):
[ 7.147289] 00:05: ttyS0 at I/O 0x3f8 (irq = 4, base_baud = 115200) is a 16550A

What am I missing? Why does it seem like serial1 doesn't exist inside the guest?

Answer 1

I found my mistake.

Because the guest startup takes so much time (at least on my machine), I always immediately used loadvm after_startup_snapshot.

I now tried to let the guest go through startup, and the redirection of /dev/ttyS1 worked perfectly.