Message-ID: <CAHqbYQsKQU2zdjJ2DQ6+MDZNk1RTkr5FcoyZzQmjJBN=TTb_jw@mail.gmail.com>
Date:	Thu, 9 Jan 2014 13:58:10 -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,
	Paolo Bonzini <pbonzini@...hat.com>
Subject: Re: [PATCH] kvm: x86: Fix debug typo error in lapic

Thanks a lot Marcelo!

On Thu, Jan 9, 2014 at 1:46 PM, Marcelo Tosatti <mtosatti@...hat.com> wrote:
> On Wed, Jan 08, 2014 at 06:35:00PM -0500, Hu Yaohui 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.
>
> Yes.
>
>> But if they are on the same pcpu, if vcpu#1 is in guest mode.
>
> If vcpu#0 and vcpu#1 are on the same pcpu, then either one of them
> is guest mode at one given moment, but not both.
>
>> 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.
>
> The TLB flush is performed synchronously, see the effect of the wait
> parameter to the smp_call_function_many function, and how that function
> is called at arch/x86/mm/tlb.c.
>
if kvm_vcpu_kick sends an host-IPI to the remote vcpu, and if that
vcpu is in guest mode.
Is it possible that the remote vcpu has accessed the stale tlb entry
before the host-IPI arrives?
If that's the case, how this problem to be solved in KVM? Thanks for your time!
>> > 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?
>
> No, it means the kvm codebase you are looking at does not support it.
>
>> Thanks for your time!
>>
>> Best Wishes,
>> Yaohui Hu
>> --
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