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

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Message-ID: <20080207123937.GC15647@elte.hu>
Date:	Thu, 7 Feb 2008 13:39:38 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	"H. Peter Anvin" <hpa@...or.com>
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.

* 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.

	Ingo
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