linux-kernel - Re: [PATCH v2 2/2] KVM: x86: Add lowest-priority support for vt-d posted-interrupts

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Date:	Mon, 21 Dec 2015 09:50:19 +0800
From:	Yang Zhang <yang.zhang.wz@...il.com>
To:	Feng Wu <feng.wu@...el.com>, pbonzini@...hat.com,
	rkrcmar@...hat.com
Cc:	kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 2/2] KVM: x86: Add lowest-priority support for vt-d
 posted-interrupts

On 2015/12/16 9:37, Feng Wu wrote:
> Use vector-hashing to deliver lowest-priority interrupts for
> VT-d posted-interrupts.
>
> Signed-off-by: Feng Wu <feng.wu@...el.com>
> ---
>   arch/x86/kvm/lapic.c | 67 ++++++++++++++++++++++++++++++++++++++++++++++++++++
>   arch/x86/kvm/lapic.h |  2 ++
>   arch/x86/kvm/vmx.c   | 12 ++++++++--
>   3 files changed, 79 insertions(+), 2 deletions(-)
>
> diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
> index e29001f..d4f2c8f 100644
> --- a/arch/x86/kvm/lapic.c
> +++ b/arch/x86/kvm/lapic.c
> @@ -854,6 +854,73 @@ out:
>   }
>
>   /*
> + * This routine handles lowest-priority interrupts using vector-hashing
> + * mechanism. As an example, modern Intel CPUs use this method to handle
> + * lowest-priority interrupts.
> + *
> + * Here is the details about the vector-hashing mechanism:
> + * 1. For lowest-priority interrupts, store all the possible destination
> + *    vCPUs in an array.
> + * 2. Use "guest vector % max number of destination vCPUs" to find the right
> + *    destination vCPU in the array for the lowest-priority interrupt.
> + */
> +struct kvm_vcpu *kvm_intr_vector_hashing_dest(struct kvm *kvm,
> +					      struct kvm_lapic_irq *irq)
> +{
> +	struct kvm_apic_map *map;
> +	struct kvm_vcpu *vcpu = NULL;
> +
> +	if (irq->shorthand)
> +		return NULL;
> +
> +	rcu_read_lock();
> +	map = rcu_dereference(kvm->arch.apic_map);
> +
> +	if (!map)
> +		goto out;
> +
> +	if ((irq->dest_mode != APIC_DEST_PHYSICAL) &&
> +			kvm_lowest_prio_delivery(irq)) {
> +		u16 cid;
> +		int i, idx = 0;
> +		unsigned long bitmap = 1;
> +		unsigned int dest_vcpus = 0;
> +		struct kvm_lapic **dst = NULL;
> +
> +
> +		if (!kvm_apic_logical_map_valid(map))
> +			goto out;
> +
> +		apic_logical_id(map, irq->dest_id, &cid, (u16 *)&bitmap);
> +
> +		if (cid >= ARRAY_SIZE(map->logical_map))
> +			goto out;
> +
> +		dst = map->logical_map[cid];
> +
> +		for_each_set_bit(i, &bitmap, 16) {
> +			if (!dst[i] && !kvm_lapic_enabled(dst[i]->vcpu)) {
> +				clear_bit(i, &bitmap);
> +				continue;
> +			}
> +		}
> +
> +		dest_vcpus = hweight16(bitmap);
> +
> +		if (dest_vcpus != 0) {
> +			idx = kvm_vector_2_index(irq->vector, dest_vcpus,
> +						 &bitmap, 16);
> +			vcpu = dst[idx-1]->vcpu;
> +		}
> +	}
> +
> +out:
> +	rcu_read_unlock();
> +	return vcpu;
> +}
> +EXPORT_SYMBOL_GPL(kvm_intr_vector_hashing_dest);
> +
> +/*
>    * Add a pending IRQ into lapic.
>    * Return 1 if successfully added and 0 if discarded.
>    */
> diff --git a/arch/x86/kvm/lapic.h b/arch/x86/kvm/lapic.h
> index 6890ef0..52bffce 100644
> --- a/arch/x86/kvm/lapic.h
> +++ b/arch/x86/kvm/lapic.h
> @@ -172,4 +172,6 @@ bool kvm_intr_is_single_vcpu_fast(struct kvm *kvm, struct kvm_lapic_irq *irq,
>   			struct kvm_vcpu **dest_vcpu);
>   int kvm_vector_2_index(u32 vector, u32 dest_vcpus,
>   		       const unsigned long *bitmap, u32 bitmap_size);
> +struct kvm_vcpu *kvm_intr_vector_hashing_dest(struct kvm *kvm,
> +					      struct kvm_lapic_irq *irq);
>   #endif
> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
> index 5eb56ed..3f89189 100644
> --- a/arch/x86/kvm/vmx.c
> +++ b/arch/x86/kvm/vmx.c
> @@ -10702,8 +10702,16 @@ static int vmx_update_pi_irte(struct kvm *kvm, unsigned int host_irq,
>   		 */
>
>   		kvm_set_msi_irq(e, &irq);
> -		if (!kvm_intr_is_single_vcpu(kvm, &irq, &vcpu))
> -			continue;
> +
> +		if (!kvm_intr_is_single_vcpu(kvm, &irq, &vcpu)) {
> +			if (!kvm_vector_hashing_enabled() ||
> +					irq.delivery_mode != APIC_DM_LOWEST)
> +				continue;
> +
> +			vcpu = kvm_intr_vector_hashing_dest(kvm, &irq);
> +			if (!vcpu)
> +				continue;
> +		}

I am a little confused with the 'continue'. If the destination is not 
single vcpu, shouldn't we rollback to use non-PI mode?

>
>   		vcpu_info.pi_desc_addr = __pa(vcpu_to_pi_desc(vcpu));
>   		vcpu_info.vector = irq.vector;
>


-- 
best regards
yang
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