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

Radim Krčmář
2016-07-13T15:14:49Z

2016-07-13 10:38+0200, Paolo Bonzini:
> 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.

KVM doesn't do the operation in parentheses since 394457a928e0 ("KVM:
x86: some apic broadcast modes does not work").

Following patches added only !ipi case (I already regret posting them).

> + *
> + * 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?

Except that one bit. (Code alone really can't explain the what happens
here.)