Hello, KVM emulates an IOAPIC that doesn't handle byte writes to its IOAPIC_REG_SELECT register, although for example the ICH10 spec[1] clearly states that this is an 8-bit register. See http://www.intel.com/content/dam/doc/datasheet/io-controller-hub-10-family-datasheet.pdf Table 13-4 on page 433.
The code in question is: http://git.kernel.org/?p=virt/kvm/kvm.git;a=blob;f=virt/kvm/ioapic.c;h=3eed61eb48675a63dd1f31b0095217ab6bc5f646;hb=HEAD#l323 This breaks IOAPIC code in OSes that adhere to the spec. I've created a small testcase[1]: $ qemu-kvm -serial stdio -kernel ioapic [26303.961804] ioapic: Unsupported size 1 IOAPIC ID 00000000 [26303.970466] ioapic: Unsupported size 1 IOAPIC VER 00000000 Done qemu: terminating on signal 2 $ qemu-kvm -no-kvm-irqchip -serial stdio -kernel ioapic IOAPIC ID 00000000 IOAPIC VER 00170011 Done qemu: terminating on signal 2 Expected behavior is that the IOAPIC register is not read as zero with KVM irqchip emulation. I would file a bug, but the kernel bugzilla seems to be down at the moment. Regards, Julian [1] http://os.inf.tu-dresden.de/~jsteckli/tmp/ioapic
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