Re: [PATCH v2] KVM: irqchip: KVM: Reduce allocation overhead in kvm_set_irq_routing()

[Yanfei Xu <isyanfei.xu@gmail.com>]

gentle ping :)

On 2025/12/26 14:27, Yanfei Xu wrote:
> In guests with many VFIO devices and MSI-X vectors, kvm_set_irq_routing()
> becomes a high-overhead operation. Each invocation walks the entire IRQ
> routing table and reallocates/frees every routing entry.
>
> As the routing table grows on each call, entry allocation and freeing
> dominate the execution time of this function. In scenarios such as VM
> live migration or live upgrade, this behavior can introduce unnecessary
> downtime.
>
> Allocate memory for all routing entries in one shot using kcalloc(),
> allowing them to be freed together with a single kfree() call.
>
> Example: On a VM with 120 vCPUs and 15 VFIO devices (virtio-net), the
> total number of calls to kzalloc and kfree is over 20000. With this
> change, it is reduced to around 30.
>
> Reported-by: Xiangfeng Cai <caixiangfeng@bytedance.com>
> Signed-off-by: Yanfei Xu <yanfei.xu@bytedance.com>
> ---
> v1->v2:
> 1. fix variable 'r' is used uninitialized when a 'if' condition is true
> 2. simplified free_irq_routing_table() by removing the iteration over the
>     entries and the hlist cleanup.
>
>   include/linux/kvm_host.h |  1 +
>   virt/kvm/irqchip.c       | 34 +++++++++++-----------------------
>   2 files changed, 12 insertions(+), 23 deletions(-)
>
> diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> index d93f75b05ae2..cc27490bef4b 100644
> --- a/include/linux/kvm_host.h
> +++ b/include/linux/kvm_host.h
> @@ -692,6 +692,7 @@ struct kvm_kernel_irq_routing_entry {
>   struct kvm_irq_routing_table {
>   	int chip[KVM_NR_IRQCHIPS][KVM_IRQCHIP_NUM_PINS];
>   	u32 nr_rt_entries;
> +	struct kvm_kernel_irq_routing_entry *entries_addr;
>   	/*
>   	 * Array indexed by gsi. Each entry contains list of irq chips
>   	 * the gsi is connected to.
> diff --git a/virt/kvm/irqchip.c b/virt/kvm/irqchip.c
> index 6ccabfd32287..56779394033d 100644
> --- a/virt/kvm/irqchip.c
> +++ b/virt/kvm/irqchip.c
> @@ -98,21 +98,10 @@ int kvm_set_irq(struct kvm *kvm, int irq_source_id, u32 irq, int level,
>   
>   static void free_irq_routing_table(struct kvm_irq_routing_table *rt)
>   {
> -	int i;
> -
>   	if (!rt)
>   		return;
>   
> -	for (i = 0; i < rt->nr_rt_entries; ++i) {
> -		struct kvm_kernel_irq_routing_entry *e;
> -		struct hlist_node *n;
> -
> -		hlist_for_each_entry_safe(e, n, &rt->map[i], link) {
> -			hlist_del(&e->link);
> -			kfree(e);
> -		}
> -	}
> -
> +	kfree(rt->entries_addr);
>   	kfree(rt);
>   }
>   
> @@ -186,6 +175,12 @@ int kvm_set_irq_routing(struct kvm *kvm,
>   	new = kzalloc(struct_size(new, map, nr_rt_entries), GFP_KERNEL_ACCOUNT);
>   	if (!new)
>   		return -ENOMEM;
> +	e = kcalloc(nr, sizeof(*e), GFP_KERNEL_ACCOUNT);
> +	if (!e) {
> +		r = -ENOMEM;
> +		goto out;
> +	}
> +	new->entries_addr = e;
>   
>   	new->nr_rt_entries = nr_rt_entries;
>   	for (i = 0; i < KVM_NR_IRQCHIPS; i++)
> @@ -193,25 +188,20 @@ int kvm_set_irq_routing(struct kvm *kvm,
>   			new->chip[i][j] = -1;
>   
>   	for (i = 0; i < nr; ++i) {
> -		r = -ENOMEM;
> -		e = kzalloc(sizeof(*e), GFP_KERNEL_ACCOUNT);
> -		if (!e)
> -			goto out;
> -
>   		r = -EINVAL;
>   		switch (ue->type) {
>   		case KVM_IRQ_ROUTING_MSI:
>   			if (ue->flags & ~KVM_MSI_VALID_DEVID)
> -				goto free_entry;
> +				goto out;
>   			break;
>   		default:
>   			if (ue->flags)
> -				goto free_entry;
> +				goto out;
>   			break;
>   		}
> -		r = setup_routing_entry(kvm, new, e, ue);
> +		r = setup_routing_entry(kvm, new, e + i, ue);
>   		if (r)
> -			goto free_entry;
> +			goto out;
>   		++ue;
>   	}
>   
> @@ -228,8 +218,6 @@ int kvm_set_irq_routing(struct kvm *kvm,
>   	r = 0;
>   	goto out;
>   
> -free_entry:
> -	kfree(e);
>   out:
>   	free_irq_routing_table(new);
>