[Qemu-devel] Re: 2.6.24 says "serial8250: too much work for irq4" a lot.

[Rob Landley <rob@landley.net>]

On Tuesday 05 February 2008 15:07:27 H. Peter Anvin wrote:
> Rob Landley wrote:
> > When running a 2.6.24 kernel built for x86-64 under qemu via serial
> > console, doing CPU-intensive things that also produce a lot of output
> > (such as compiling software) tends to produce the error message in the
> > title.
> >
> > Anybody have a clue why?  It doesn't seem to cause an actual problem, but
> > it's kind of annoying.
> >
> > (If it's a qemu issue, I can go bother them.  It's possible that qemu
> > isn't delivering interrupts as often as it expects, since that's limited
> > by the granularity of the host timer; I know the clock in qemu can run a
> > bit slow because it only gets clock interrupts when the host system isn't
> > too busy to schedule the emulator.  But this doesn't usually cause a
> > problem.  I _think_ the message is just a "this should never happen" type
> > warning, which is happening to me.  But I break stuff. :)
>
> This is because Qemu spews data to the serial port without any rate
> limiting; this causes the in-kernel serial port driver to think the port
> is stuck.  The serial port emulation needs to make it possible to drain
> the virtual FIFO every now and then, as opposed to filling it again
> immediately.
>
> 	-hpa

Thanks.  cc'd the right list on this one. :)

Rob
-- 
"One of my most productive days was throwing away 1000 lines of code."
  - Ken Thompson.