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Re: [Xen-users] Serial Console oddity

On Tue, 16 Aug 2005, Derrik Pates wrote:

> Gino LV. Ledesma wrote:
> > Has anyone encountered a situation where the boot output stops after
> > switchroot is made? What happens in my instal is that xen's serial
> > console output stops at the moment the WARNING message about tls is
> > made, and the very last message on screen/serial console is
> > "Continuing...." The computer proceeds to boot, though nothing is
> > shown on the console. So I don't get to access the xen host over
> > serial console.
> >     module /vmlinuz- ro root=/dev/VolGroup01/LogVolRoot
> > nousb console=xencons0 console=tty0
> This might have something to do with it. It's not "xencons0", XenoLinux
> sees it as ttyS0. Also, if you want the output of init to go to ttyS0
> (so your initscripts' startup messages show up there), specify it
> second, not first.
> > 8250 Serial Driver compiled in.
> The Xen hyperkernel has to take possession of the serial port, so
> building in the 8250/16550 serial driver will break things. You
> shouldn't enable it.

Actually, The situation seems pretty simple. It just isn't documented, so
it took me a while to figure it out.

If you compile in the "real linux" serial drivers and/or console, then you
break sharing the linux console with xen. This is because (as Gino just
pointed out) that xen needs to take the serial port, and thus needs to
"emulate" it for the linux console driver.

If you have a scenario where you really do want to talk to a serial port
from linux, then you need the serial driver in the kernel, and you need to
either give up having a serial port for xen, or a serial port for linux.

Actually, there is a bit of a work around. For initial setup purposes, you
can have two grub configurations. One which gives the serial port to xen,
and the other which gives the port to the linux kernel. That way you make
sure you have xen up and happy, and then switch to using the config that
lets you see your dom0 linux boot.

Hope that helps someone. For me, with our remote servers, I _had_ to
figure it out.


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