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

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Message-ID: <CAHqbYQu0rYfviVMJhgeEnBKYri5OVMA7XEp2ZCVGpvsiTYRJGg@mail.gmail.com>
Date:	Wed, 8 Jan 2014 18:35:00 -0500
From:	Hu Yaohui <loki2441@...il.com>
To:	Marcelo Tosatti <mtosatti@...hat.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

Thanks a lot Marcelo!

On Wed, Jan 8, 2014 at 6:25 PM, Marcelo Tosatti <mtosatti@...hat.com> wrote:
> 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!
>
I am using kvm-kmod-3.2
> 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.
>
if vcpu#1 is not on the same pcpu as the vcpu#0, a host-IPI will be sent.
But if they are on the same pcpu, if vcpu#1 is in guest mode. Then the
guest tlb flush IPI
will wait until the next vcpu#1 vmexit. If that's the case. they are
some time that the tlb entry has been
invalidated in vcpu#0, but the corresponding entry in vcpu#1 could
still been accessed, which seems cause some problem.

> 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).
>
>
I did not find this function (vmx_deliver_posted_interrupt) in my kvm
kernel module.
Does that mean my hardware doesn't support APIC virtualization?

Thanks for your time!

Best Wishes,
Yaohui Hu
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