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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