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

From: H. Peter Anvin
Date: Thu Feb 07 2008 - 12:37:45 EST


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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