Re: [Xen-devel] Serial console hangs with Linux 2.6.20 HVM guest

[Ian Jackson <Ian.Jackson@xxxxxxxxxxxxx>]

Anders Kaseorg writes ("Re: [Xen-devel] Serial console hangs with Linux 2.6.20 
HVM guest"):
> This breakpoint is triggered for all messages printed by the kernel,
> which always showed up with no delay; but it is only occasionally
> triggered for strings printed by userspace, even after forcing those
> strings to show up by sending keystrokes.

That's interesting ....

Anders Kaseorg writes ("Re: [Xen-devel] Serial console hangs with Linux 2.6.20 
HVM guest"):
> In case this is more interesting, here is a log of all the breakpoints
> hit during a Linux boot, up to the point where it hangs:
> 
> http://web.mit.edu/andersk/Public/xen-serial-log

.... looking at this, it seems like we definitely don't have the bug
that the Linux kernel is trying to work around.  In each of the cases
where the IER was written there by the kernel, an interrupt was
already pending.

But since the Linux change is just to have a failsafe timer for lost
interrupts, any other kind of lost interrupt situation might cause it.
Perhaps the kernel is expecting us to deassert and reassert the
interrupt in some situation when we aren't; since the interrupts are
converted from level to edge triggered it's possible that we are
failing to _deassert_ the interrupt when we should.  I'm just
speculating here at the moment ...

Can you recompile your qemu-dm with
  #define DEBUG_SERIAL
near the top of serial.c uncommented, and try booting up and get it to
the point where some output is supposed to have come out but has got
stuck, and then send me your /var/log/xen/qemu-dm-<whatever>.log ?

If it's short post it to the list; otherwise put it up on a webpage or
email it to me privately.

Thanks,
Ian.

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