linux-kernel - Re: [PATCH 2/3] kvm: x86: do not use KVM_REQ

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Date:   Wed, 28 Sep 2016 13:50:29 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     "Wu, Feng" <feng.wu@...el.com>,
        "Michael S. Tsirkin" <mst@...hat.com>
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "yang.zhang.wz@...il.com" <yang.zhang.wz@...il.com>,
        "rkrcmar@...hat.com" <rkrcmar@...hat.com>
Subject: Re: [PATCH 2/3] kvm: x86: do not use KVM_REQ_EVENT for APICv
 interrupt injection

On 28/09/2016 13:40, Wu, Feng wrote:
> IIUIC, the issue you describe above is that IPI for posted-interrupts may be
> issued between
> 
> vcpu->mode = IN_GUEST_MODE;
> 
> and
> 
> local_irq_disable();
> 
> But if that really happens, we will call kvm_vcpu_kick() in
> vmx_deliver_posted_interrupt(), hence the vcpu->mode will be changed
> to EXITING_GUEST_MODE, then we will goto cancel_injection in
> vcpu_enter_guest, so the posted-interrupt will be delivered to guest
> in the next vmentry. Seems I cannot see the problem. Do I miss something?

No, if that happens kvm_trigger_posted_interrupt returns true, hence
kvm_vcpu_kick is not called.  With the fix, the IPI is processed as soon
as the guest enters non-root mode, and the interrupt is injected.

The other issue occurs when the IPI is sent between

                        kvm_x86_ops->hwapic_irr_update(vcpu,
                                kvm_lapic_find_highest_irr(vcpu));

and

	vcpu->mode = IN_GUEST_MODE;

In this case, kvm_vcpu_kick is called but it (correctly) doesn't do
anything because it sees vcpu->mode == OUTSIDE_GUEST_MODE.  Then the
guest is entered with PIR.ON, but the PI interrupt is not pending and
hence the interrupt is never delivered to the guest.  The fix for this
is to move the RVI update after IN_GUEST_MODE.  Then the source CPU uses
the posted interrupt IPI instead of kvm_cpu_kick, and everything works.

Paolo