Re: [Xen-ia64-devel] RE: [PATCH] fixed serial console support

[Alex Williamson <alex.williamson@xxxxxx>]

On Thu, 2005-12-08 at 17:50 +0200, Tristan Gingold wrote:
> >
> > FYI, I tried setting CONFIG_VT=y for domU.  No console output
> > after "Freeing unused kernel memory" and then dom0 crashed!
> I have just tried again.
> For dom0: console does not work (I am searching the reason).
> For domU: No console output after "Freeing unused...", but no crash.
> Dom0 kernel = domU kernel.
> 
> The only bug is why /dev/console does not work.

   I debugged a little further and I think I know the problem.  On dom0,
the in kernel xen console registers w/ the tty layer as a serial device,
claiming standard serial major/minor numbers.  On domU domains, this
same driver tries to claim the major/minor numbers used by the vt
driver.  The problem is simply that we cannot have 2 drivers trying to
claim the same device ranges.  Typically the normal in kernel drivers
register first and locks out the xen console driver.  Because of this,
it's standard practice that the xen0 config does not contain the 8250
serial driver and the xenU config does not contain vt support.

   I sent a note to xen-devel about this and Ian responded, but I
haven't gotten his suggestion to work yet.  If we specify
xenconsole=ttySX where X is something above the device range the 8250
driver claims, it should work (my few attempts crashed dom0).  I think
the same can be specified for domU domains to get it to also use a
serial virtual device.  Alternatively a xen console driver could be
created that has it's own major/minor device numbers assigned so there's
no conflict.  I saw there was also a virtual frame buffer in the works
on x86.  While not ideal for a server kind of system, it's perhaps
something we could leverage, and it would be fun if it worked anyway.
Thanks,

        Alex
-- 
Alex Williamson                             HP Linux & Open Source Lab


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