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

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Date:   Wed, 28 Sep 2016 10:04:01 +0000
From:   "Wu, Feng" <feng.wu@...el.com>
To:     Paolo Bonzini <pbonzini@...hat.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>
CC:     "yang.zhang.wz@...il.com" <yang.zhang.wz@...il.com>,
        "mst@...hat.com" <mst@...hat.com>,
        "rkrcmar@...hat.com" <rkrcmar@...hat.com>,
        "Wu, Feng" <feng.wu@...el.com>
Subject: RE: [PATCH 2/3] kvm: x86: do not use KVM_REQ_EVENT for APICv
 interrupt injection



> -----Original Message-----
> From: Paolo Bonzini [mailto:paolo.bonzini@...il.com] On Behalf Of Paolo
> Bonzini
> Sent: Wednesday, September 28, 2016 5:20 AM
> To: linux-kernel@...r.kernel.org; kvm@...r.kernel.org
> Cc: yang.zhang.wz@...il.com; Wu, Feng <feng.wu@...el.com>;
> mst@...hat.com; rkrcmar@...hat.com
> Subject: [PATCH 2/3] kvm: x86: do not use KVM_REQ_EVENT for APICv interrupt
> injection
> 
> Since bf9f6ac8d749 ("KVM: Update Posted-Interrupts Descriptor when vCPU
> is blocked", 2015-09-18) the posted interrupt descriptor is checked
> unconditionally for PIR.ON.  Therefore we don't need KVM_REQ_EVENT to
> trigger the scan and, if NMIs or SMIs are not involved, we can avoid
> the complicated event injection path.

But the following code still remains in the KVM_REQ_EVENT checking part:

if (kvm_lapic_enabled(vcpu)) {
	update_cr8_intercept(vcpu);
	kvm_lapic_sync_to_vapic(vcpu);
}

Does this matter?

Thanks,
Feng

> 
> However, there is a race between vmx_deliver_posted_interrupt and
> vcpu_enter_guest.  Fix it by disabling interrupts before vcpu->mode is
> set to IN_GUEST_MODE.
> 
> Calling kvm_vcpu_kick if PIR.ON=1 is also useless, though it has been
> there since APICv was introduced.
> 
> Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
> ---
>  arch/x86/kvm/lapic.c | 2 --
>  arch/x86/kvm/vmx.c   | 8 +++++---
>  arch/x86/kvm/x86.c   | 9 +++++++--
>  3 files changed, 12 insertions(+), 7 deletions(-)
> 
> diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
> index 63a442aefc12..be8b7ad56dd1 100644
> --- a/arch/x86/kvm/lapic.c
> +++ b/arch/x86/kvm/lapic.c
> @@ -356,8 +356,6 @@ void kvm_apic_update_irr(struct kvm_vcpu *vcpu, u32
> *pir)
>  	struct kvm_lapic *apic = vcpu->arch.apic;
> 
>  	__kvm_apic_update_irr(pir, apic->regs);
> -
> -	kvm_make_request(KVM_REQ_EVENT, vcpu);
>  }
>  EXPORT_SYMBOL_GPL(kvm_apic_update_irr);
> 
> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
> index b33eee395b00..207b9aa32915 100644
> --- a/arch/x86/kvm/vmx.c
> +++ b/arch/x86/kvm/vmx.c
> @@ -4844,9 +4844,11 @@ static void vmx_deliver_posted_interrupt(struct
> kvm_vcpu *vcpu, int vector)
>  	if (pi_test_and_set_pir(vector, &vmx->pi_desc))
>  		return;
> 
> -	r = pi_test_and_set_on(&vmx->pi_desc);
> -	kvm_make_request(KVM_REQ_EVENT, vcpu);
> -	if (r || !kvm_vcpu_trigger_posted_interrupt(vcpu))
> +	/* If a previous notification has sent the IPI, nothing to do.  */
> +	if (pi_test_and_set_on(&vmx->pi_desc))
> +		return;
> +
> +	if (!kvm_vcpu_trigger_posted_interrupt(vcpu))
>  		kvm_vcpu_kick(vcpu);
>  }
> 
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 3ee8a91a78c3..604cfbfc5bee 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -6658,6 +6658,13 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
>  	kvm_x86_ops->prepare_guest_switch(vcpu);
>  	if (vcpu->fpu_active)
>  		kvm_load_guest_fpu(vcpu);
> +
> +	/*
> +	 * Disable IRQs before setting IN_GUEST_MODE, so that
> +	 * posted interrupts with vcpu->mode == IN_GUEST_MODE
> +	 * always result in virtual interrupt delivery.
> +	 */
> +	local_irq_disable();
>  	vcpu->mode = IN_GUEST_MODE;
> 
>  	srcu_read_unlock(&vcpu->kvm->srcu, vcpu->srcu_idx);
> @@ -6671,8 +6678,6 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
>  	 */
>  	smp_mb__after_srcu_read_unlock();
> 
> -	local_irq_disable();
> -
>  	if (vcpu->mode == EXITING_GUEST_MODE || vcpu->requests
>  	    || need_resched() || signal_pending(current)) {
>  		vcpu->mode = OUTSIDE_GUEST_MODE;
> --
> 1.8.3.1
>