linux-kernel - Re: [PATCH] kvm: x86: Fix debug typo error in lapic

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Message-ID: <20140108232527.GA17874@amt.cnet>
Date:	Wed, 8 Jan 2014 21:25:27 -0200
From:	Marcelo Tosatti <mtosatti@...hat.com>
To:	Hu Yaohui <loki2441@...il.com>
Cc:	Chen Fan <chen.fan.fnst@...fujitsu.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	kvm <kvm@...r.kernel.org>, gleb@...hat.com, pbonzini@...hat.com
Subject: Re: [PATCH] kvm: x86: Fix debug typo error in lapic

On Wed, Jan 08, 2014 at 06:14:15PM -0500, Hu Yaohui wrote:
> Hi guys,
> I think you should be pretty familiar with lapic. I would really
> appreciate it if someone could shed some lights on my problem
> regarding Guest TLB flush IPI.
> Supposed we get two vcpus 0 and 1.
> When vcpu#0 wants to invalidate the tlb entry on vcpu#1. An IPI will
> be generated by lapic on vcpu#0 by writing to ICR which will cause a
> vmexit.
> apic_send_ipi->kvm_irq_delivery_to_apic->kvm_apic_set_irq->__apic_accept_irq
> In __apic_accept_irq, it will call kvm_make_request, kvm_vcpu_kick.
> If vcpu#1 in guest mode, how can it receives this IPI immediately, or
> the stale tlb entry could be accessed. Thanks for your time!

Two possibilities: 

2) Hardware does not support APIC virtualization: kvm_vcpu_kick sends an
host-IPI to the remote vcpu, and if that vcpu is in guest mode, a VM-exit
(exit reason: external interrupt) will be triggered due to the host-IPI.
Then on VM-entry (inject_pending_event) the guest-IPI is injected.

2) Host CPU supports APIC virtualization (see commit 83d4c286931c and
Intel's documentation): 
A bit is set in the posted interrupt section, and a special host-IPI is
delivered to the target cpu where the guest vcpu is scheduled 
(vmx_deliver_posted_interrupt) which causes the hardware to 
inject the vector (without VM-exit).

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