Re: 2.6.24 says "serial8250: too much work for irq4" a lot.

[H. Peter Anvin]

Ingo Molnar wrote:
> * H. Peter Anvin <hpa@xxxxxxxxx> wrote:
>
> > > (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.
>
> actually, the way i solved it for qemu+KVM+paravirt was to just turn off 
> this rather silly check in the serial driver if inside a paravirt guest. 
> When we are emulated then the serial 'hardware' is totally reliable and 
> we should just trust it. That way i never dropped a single bit of kernel 
> log output again.

Yes, but keying that on paravirt is silly in the extreme.  After all, 
there is no need for this to be paravirtualized.

	-hpa
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