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

[Marcelo Tosatti <mtosatti@redhat.com>]

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@redhat.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.

> > 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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