linux-kernel - Re: [PATCH] kvm: nVMX: fix entry with pending interrupt if APICv is enabled

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Date:   Wed, 3 Oct 2018 07:53:24 -0700
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        Nikita Leshchenko <nikita.leshchenko@...cle.com>,
        Liran Alon <liran.alon@...cle.com>,
        Radim Krčmář <rkrcmar@...hat.com>
Subject: Re: [PATCH] kvm: nVMX: fix entry with pending interrupt if APICv is
 enabled

On Wed, Oct 03, 2018 at 01:47:47PM +0200, Paolo Bonzini wrote:
> Commit b5861e5cf2fcf83031ea3e26b0a69d887adf7d21 introduced a check on
> the interrupt-window and NMI-window CPU execution controls in order to
> inject an external interrupt vmexit before the first guest instruction
> executes.  However, when APIC virtualization is enabled the host does not
> need a vmexit in order to inject an interrupt at the next interrupt window;
> instead, it just places the interrupt vector in RVI and the processor will
> inject it as soon as possible.  Therefore, on machines with APICv it is
> not enough to check the CPU execution controls: the same scenario can also
> happen if RVI>0.
> 
> Fixes: b5861e5cf2fcf83031ea3e26b0a69d887adf7d21
> Cc: Nikita Leshchenko <nikita.leshchenko@...cle.com>
> Cc: Sean Christopherson <sean.j.christopherson@...el.com>
> Cc: Liran Alon <liran.alon@...cle.com>
> Cc: Radim Krčmář <rkrcmar@...hat.com>
> Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
> ---
>  arch/x86/kvm/vmx.c | 16 +++++++++++-----
>  1 file changed, 11 insertions(+), 5 deletions(-)
> 
> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
> index 6ef2d5b139b9..c0c7689f0049 100644
> --- a/arch/x86/kvm/vmx.c
> +++ b/arch/x86/kvm/vmx.c
> @@ -10280,6 +10280,11 @@ static void vmx_hwapic_isr_update(struct kvm_vcpu *vcpu, int max_isr)
>  	}
>  }
>  
> +static u8 vmx_get_rvi(void)
> +{
> +	return vmcs_read16(GUEST_INTR_STATUS) & 0xff;
> +}
> +
>  static void vmx_set_rvi(int vector)
>  {
>  	u16 status;
> @@ -12593,10 +12598,13 @@ static int enter_vmx_non_root_mode(struct kvm_vcpu *vcpu, u32 *exit_qual)
>  	struct vmcs12 *vmcs12 = get_vmcs12(vcpu);
>  	bool from_vmentry = !!exit_qual;
>  	u32 dummy_exit_qual;
> -	u32 vmcs01_cpu_exec_ctrl;
> +	bool evaluate_pending_interrupts;
>  	int r = 0;
>  
> -	vmcs01_cpu_exec_ctrl = vmcs_read32(CPU_BASED_VM_EXEC_CONTROL);
> +	evaluate_pending_interrupts = vmcs_read32(CPU_BASED_VM_EXEC_CONTROL) &
> +		(CPU_BASED_VIRTUAL_INTR_PENDING | CPU_BASED_VIRTUAL_NMI_PENDING);
> +	if (enable_apicv && kvm_vcpu_apicv_active(vcpu))

Isn't enable_apicv redundant with kvm_vcpu_apicv_active()?  And since
getting RVI requires a VMREAD, I think it would make sense to only
fall into this code if !evaluate_pending_interrupts, e.g.:

	if (!evaluate_pending_interrupts && kvm_vcpu_apicv_active(vcpu))
		evaluate_pending_interrupts |= vmx_get_rvi() > 0;

> +		evaluate_pending_interrupts |= vmx_get_rvi() > 0;
>  
>  	enter_guest_mode(vcpu);
>  
> @@ -12650,10 +12658,8 @@ static int enter_vmx_non_root_mode(struct kvm_vcpu *vcpu, u32 *exit_qual)
>  	 * instead. Thus, we force L0 to perform pending event
>  	 * evaluation by requesting a KVM_REQ_EVENT.
>  	 */
> -	if (vmcs01_cpu_exec_ctrl &
> -		(CPU_BASED_VIRTUAL_INTR_PENDING | CPU_BASED_VIRTUAL_NMI_PENDING)) {
> +	if (evaluate_pending_interrupts)
>  		kvm_make_request(KVM_REQ_EVENT, vcpu);
> -	}
>  
>  	/*
>  	 * Note no nested_vmx_succeed or nested_vmx_fail here. At this point
> -- 
> 1.8.3.1
>