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

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Date:	Thu, 07 Feb 2008 09:37:12 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Ingo Molnar <mingo@...e.hu>
CC:	Rob Landley <rob@...dley.net>, linux-kernel@...r.kernel.org
Subject: Re: 2.6.24 says "serial8250: too much work for irq4" a lot.

Ingo Molnar wrote:
> * H. Peter Anvin <hpa@...or.com> 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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