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

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Date:   Wed, 28 Sep 2016 02:07:11 +0300
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        yang.zhang.wz@...il.com, feng.wu@...el.com, rkrcmar@...hat.com
Subject: Re: [PATCH 2/3] kvm: x86: do not use KVM_REQ_EVENT for APICv
 interrupt injection

On Tue, Sep 27, 2016 at 11:20:12PM +0200, Paolo Bonzini wrote:
> 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.
> 
> 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.

Could you describe the race a bit more please?
I'm surprised that locally disabling irqs
fixes a race with a different CPUs.

> 
> 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))

so the logic here was reversed - kick was not called
when PIR=OFF previously. It's a bug but what was it's effect?
I'm guessing - extra latency where VCPU won't get
woken up correctly?

> +	/* 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
>