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

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Date:	Thu, 9 Jan 2014 11:28:40 -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

Hi Macelo,
I am sorry to bother you again. In your first possibility,
kvm_vcpu_kick sends an host-IPI to the remote vcpu,
and if that vcpu is in guest mode, a VM-exit will be triggered due to
the host-IPI. My question is if the vcpu has
accessed the stale tlb entry before the host-IPI arrives, what will
happen?  Thanks for your time!

Best Wishes,
Yaohui Hu


On Wed, Jan 8, 2014 at 6:35 PM, Hu Yaohui <loki2441@...il.com> wrote:
> 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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