Re: [PATCH] KVM: IOAPIC: Do not check polarity

[Avi Kivity <avi@redhat.com>]

Bernhard Kohl wrote:
> NSN's proprietary OS DMX heavily depends on a correct BIOS MP table.
> From there it also gets the information for initialization of PCI
> devices. We defined the polarity and trigger mode as for standard
> PCI devices "low level". The OS programs the ioapic the same way.
>
> Nothing else in QEMU's and KVM's IRQ, PIC, IOAPIC and LAPIC code
> cares about interrupt polarity. The "level" is always used as a
> logical value. So we must not change the level here. If we do so,
> PCI low level interrupts are not fired if the guest raises the
> interrupt (level=1), but first if it is reset again (level=0). This
> can result in infinite waiting for a single interupt. For example
> the DMX guest waits for the link up interupt of the E1000
> ethernet device before enabling the TX and RX interupts of
> this device. E1000 emulation sets level=1, no interrupt is fired,
> deadlock.
>
> This does not happen if we use the -no-kvm-irqchip option.
> This means that qemu's ioapic does not care about the polarity and
> kvm's ioapic shouldn't do either.
>   

While qemu uses active-high interrupts throughout, kvm is meant to be 
useful with other userspace (or for emulating other chipsets) and thus 
needs correct polarity emulation.

If the guest programs polarity to active-low, this indicates either a 
guest bug or a bios bug.  Is it possible the bios does not communicate 
the fact that pci interrupts are active high correctly?


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