Re: [Qemu-discuss] How to commucation by serial ports between host and target?

[Mike Lovell]

On 03/28/2012 08:02 PM, Parmenides wrote:
> HI,
>
>      When debugging the linux kernel with kgdb, it is necessary to
> commucation between the host and the target. I started the qemu with:
>
>                qemu -m 64M -kernel arch/x86/boot/bzImage -initrd
> ~/image.cpio.gz  -net nic -net tap,ifname=tap0 -vnc :1 -serial
> /dev/ttyS0
>
> and think the target will redirect its serial port to the host's
> /dev/ttyS0. I set the serial ports
>
>                stty ispeed 115200 ospeed 115200 -F /dev/ttyS0
>
> in the target, and
>
>                stty ispeed 115200 ospeed 115200 -F /dev/ttyS0
>
> Then,
>
>                cat /dev/ttyS0
>
> in the target, and
>
>                echo hello>  /dev/ttyS0
>
> But, it seems that the commucation does not work. How to make the host
> and the target commucation by their serial ports? Thx.

i think the problem is that you are using the physical serial port on 
the host instead of using a virtual device. unless you were to have a 
loopback device on the physical serial port of the host, the bits going 
out of it aren't going to be able to be read by the same device.

try using '-serial pty' on the guest. that will have qemu get a new pty 
allocated to it that it will use for serial communication. it will say 
something like "char device redirected to /dev/pts/11" when qemu is 
started. you can then read and write from the filename given and the 
result will be sent to and read from the guest. here is what i just tried.

host:
qemu-system-x86_64 -enable-kvm -m 2048 -smp 2 -serial pty -drive 
file=/my/vm/disk.qcow2
char device redirected to /dev/pts/11

guest:
cat /dev/ttyS0

host:
echo 'this is a test' > /dev/pts/11

'this is a test' then shows up in the guest. so give that a try. there 
are many other options you can use for serial ports including sockets 
and named pipes. the qemu man page has info about those.

mike