Re: [PATCH v3 12/14] KVM: x86: add a flag to disable KVM x2apic broadcast quirk

Paolo Bonzini
2016-07-13T08:39:06Z

On 12/07/2016 22:09, Radim Krčmář wrote:
> @@ -619,14 +619,17 @@ static bool kvm_apic_match_logical_addr(struct kvm_lapic *apic, u32 mda)
> /* KVM APIC implementation has two quirks
> * - dest always begins at 0 while xAPIC MDA has offset 24,
> * - IOxAPIC messages have to be delivered (directly) to x2APIC.
> + *
> + * The broadcast quirk can be disabled with KVM_CAP_X2APIC_API.

--verbose version:

/* KVM APIC implementation has two quirks
- * - dest always begins at 0 while xAPIC MDA has offset 24,
- * - IOxAPIC messages have to be delivered (directly) to x2APIC.
+ * - dest always begins at 0 while xAPIC MDA has offset 24. This is
+ * just a quirk in the API and is not problematic.
*
- * The broadcast quirk can be disabled with KVM_CAP_X2APIC_API.
+ * - in-kernel IOAPIC messages have to be delivered directly to
+ * x2APIC, because the kernel does not support interrupt remapping.
+ * In order to support broadcast without interrupt remapping, x2APIC
+ * rewrites the destination of non-IPI messages (and also of IPIs sent
+ * from xAPIC-mode LAPICs) from APIC_BROADCAST to X2APIC_BROADCAST.
+ *
+ * The broadcast quirk can be disabled with KVM_CAP_X2APIC_API. This is
+ * important when userspace wants to use x2APIC-format MSIs, because
+ * APIC_BROADCAST (0xff) is a legal route for "cluster 0, CPUs 0-7".
*/

Sounds good?

Paolo