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

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Message-ID: <4b30aa90-24bb-c5d0-18c0-b1fb4a62014f@redhat.com>
Date:	Wed, 13 Jul 2016 10:38:45 +0200
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Radim Krčmář <rkrcmar@...hat.com>,
	linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Cc:	"Lan, Tianyu" <tianyu.lan@...el.com>,
	Igor Mammedov <imammedo@...hat.com>,
	Jan Kiszka <jan.kiszka@....de>, Peter Xu <peterx@...hat.com>,
	Yang Zhang <yang.zhang.wz@...il.com>
Subject: Re: [PATCH v3 12/14] KVM: x86: add a flag to disable KVM x2apic
 broadcast quirk



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