Re: [Qemu-devel] [RFC] IRQ acknowledge on MIPS

[Marius Groeger <mgroeger@sysgo.com>]

On Tue, 23 Jan 2007, Aurelien Jarno wrote:

> There is currently a bug concerning the IRQ acknowlege on the MIPS
> system emulation. It concerns both the QEMU and Malta boards, though it
> is only detectable with a 2.4 kernel and thus on the Malta board. The
> symptom is a storm of "We got a spurious interrupt from PIIX4."
> 
> This is due to the kernel requesting the interrupt number from the 
> i8259A where no interrupt is waiting. In such a case the i8259A answers
> by an IRQ 7.
> 
> When an hardware interrupt occurs, the i8259A memorizes the interrupt
> and sends it to the MIPS CPU. This is done via the pic_irq_request()
> function. The result is that the bit 10 of the CP0 Cause register is 
> set to one (interrupt 2). But when the interrupt is finished, the i8259a
> registers IRR and ISR are cleared, but not the CP0 Cause register. The
> CPU always thinks there is an interrupt to serve, which is wrong.

I can confirm this issue. For our (custom) OS I worked around this by 
manualy clearing CP0 Cause (even though I think I shouldn't be allowed 
to do that since CP0:IP[7-2] are read-only, but that's another 
story... ;-)

> Does anyone has an idea of a sane implementation for that? It seems
> only the MIPS platform has to clear a register of the CPU when an
> interrupt is finished.

What about passing another hook to pic_init which, if set, would be 
called when no more interrupts are pending? Would that be too specific 
this problem to be an acceptable solution?

Regards,
Marius

-- 
Marius Groeger <mgroeger@sysgo.com>
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