Re: [PATCH] KVM: x86: Inject pending interrupt even if pending nmi exist

[Jan Kiszka <jan.kiszka@siemens.com>]

On 2016-03-22 06:28, Yuki Shibuya wrote:
> Non maskable interrupts (NMI) are preferred to interrupts in current
> implementation. If a NMI is pending and NMI is blocked by the result
> of nmi_allowed(), pending interrupt is not injected and
> enable_irq_window() is not executed, even if interrupts injection is
> allowed.
> 
> In old kernel (e.g. 2.6.32), schedule() is often called in NMI context.
> In this case, interrupts are needed to execute iret that intends end
> of NMI. The flag of blocking new NMI is not cleared until the guest
> execute the iret, and interrupts are blocked by pending NMI. Due to
> this, iret can't be invoked in the guest, and the guest is starved
> until block is cleared by some events (e.g. canceling injection).
> 
> This patch injects pending interrupts, when it's allowed, even if NMI
> is blocked. And, if NMI pending count == 2, NMI is not blocked and an
> interrupt is pending, NMI pending count is decremented to execute
> enable_irq_window().

The first part I understand and agree with. But the part after "And"
worries me still: if we can simply decrement that pending counter once
again - why can't we just clear it to 0 in the first place?

BTW, some inline documentation of this tricky logic would probably be good.

Jan

> 
> Signed-off-by: Yuki Shibuya <shibuya.yk@ncos.nec.co.jp>
> ---
>  arch/x86/kvm/x86.c | 12 +++++++-----
>  1 file changed, 7 insertions(+), 5 deletions(-)
> 
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 7236bd3..1373627 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -6087,12 +6087,14 @@ static int inject_pending_event(struct kvm_vcpu *vcpu, bool req_int_win)
>  	}
>  
>  	/* try to inject new event if pending */
> -	if (vcpu->arch.nmi_pending) {
> -		if (kvm_x86_ops->nmi_allowed(vcpu)) {
> +	if (vcpu->arch.nmi_pending && kvm_x86_ops->nmi_allowed(vcpu)) {
> +		--vcpu->arch.nmi_pending;
> +		vcpu->arch.nmi_injected = true;
> +		kvm_x86_ops->set_nmi(vcpu);
> +
> +		if (vcpu->arch.nmi_pending && kvm_cpu_has_injectable_intr(vcpu)
> +					&& kvm_x86_ops->interrupt_allowed(vcpu))
>  			--vcpu->arch.nmi_pending;
> -			vcpu->arch.nmi_injected = true;
> -			kvm_x86_ops->set_nmi(vcpu);
> -		}
>  	} else if (kvm_cpu_has_injectable_intr(vcpu)) {
>  		/*
>  		 * Because interrupts can be injected asynchronously, we are
> 

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