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Re: [Xen-devel] Re: xen hg tree from yesterda hangs when kernel console is directed to ttyS0

> Hi Keir,
> Thanks for the heads-up.  I'll certainly give it a try.
> As this hang appears during boot, it's not clear how much reducing
> spinlock contention will help.  At times, it's not even clear that
> I make it into __start_xen.  Certainly, when I get a hang, I
> never make it out of __start_xen/init_e820/print_e820_memory_map.
> As it often stops in the middle of print_e820_memory_map
> output, that's what lead to the suspicion about serial lines.
> Clearly, though, it's merely a suspicion/guess.

That's a very interesting data point. It means that the problem you
are seeing has nothing to do with asynchronous interrupt-driven
transmit. At that point during boot characters are synchronously
written to the transmit holding register when the status register
indicates it is empty. It's all amazingly straightforward. And of
course there can be no SMP issues either.

 -- Keir

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