linux-kernel - Re: [PATCH v3 3/4] KVM: x86: Add lowest-priority support for vt-d posted-interrupts

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Date:	Thu, 21 Jan 2016 21:16:36 +0100
From:	Radim Krčmář <rkrcmar@...hat.com>
To:	Feng Wu <feng.wu@...el.com>
Cc:	pbonzini@...hat.com, linux-kernel@...r.kernel.org,
	kvm@...r.kernel.org
Subject: Re: [PATCH v3 3/4] KVM: x86: Add lowest-priority support for vt-d
 posted-interrupts

2016-01-20 09:42+0800, Feng Wu:
> Use vector-hashing to deliver lowest-priority interrupts for
> VT-d posted-interrupts. This patch extends kvm_intr_is_single_vcpu()
> to support lowest-priority handling.
> 
> Signed-off-by: Feng Wu <feng.wu@...el.com>
> ---
> v3:
> - Remove unnecessary check in fast irq delivery patch
> - print a error message only once for each guest when we find hardware
>   disabled LAPIC during interrupt injection.
> 
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> @@ -1316,8 +1316,8 @@ int x86_set_memory_region(struct kvm *kvm, int id, gpa_t gpa, u32 size);
>  bool kvm_vcpu_is_reset_bsp(struct kvm_vcpu *vcpu);
>  bool kvm_vcpu_is_bsp(struct kvm_vcpu *vcpu);
>  
> -bool kvm_intr_is_single_vcpu(struct kvm *kvm, struct kvm_lapic_irq *irq,
> -			     struct kvm_vcpu **dest_vcpu);
> +bool kvm_intr_can_posting(struct kvm *kvm, struct kvm_lapic_irq *irq,

I prefer the original one;  I think it's better to describe function
than intent in names -- intention should be obvious from its use.

> +			  struct kvm_vcpu **dest_vcpu);
>  
>  void kvm_set_msi_irq(struct kvm_kernel_irq_routing_entry *e,
>  		     struct kvm_lapic_irq *irq);
> diff --git a/arch/x86/kvm/irq_comm.c b/arch/x86/kvm/irq_comm.c
> @@ -300,13 +300,13 @@ out:
>  	return r;
>  }
>  
> -bool kvm_intr_is_single_vcpu(struct kvm *kvm, struct kvm_lapic_irq *irq,
> -			     struct kvm_vcpu **dest_vcpu)
> +bool kvm_intr_can_posting(struct kvm *kvm, struct kvm_lapic_irq *irq,
> +		struct kvm_vcpu **dest_vcpu)
>  {
>  	int i, r = 0;
>  	struct kvm_vcpu *vcpu;
>  
> -	if (kvm_intr_is_single_vcpu_fast(kvm, irq, dest_vcpu))
> +	if (kvm_intr_can_posting_fast(kvm, irq, dest_vcpu))
>  		return true;

There is one pitfall:  xAPIC flat logical broadcast returns false, but
lowest priority is defined for it (practically isn't a broadcast) and
the rest of this function doesn't check for lowest priority, so the
interrupt won't be posted.

We could modify our _fast functions to cover 0xff in flat logical, but
ignoring this case isn't bad either ... it can happen only with 8 VCPU
guests.  I'd just comment that we did it on purpose. :)

>  
>  	kvm_for_each_vcpu(i, vcpu, kvm) {

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