Re: [PATCH] kvm: irqfd: avoid update unmodified entries of the routing

[Paolo Bonzini <pbonzini@redhat.com>]

On 27/08/21 10:00, Longpeng(Mike) wrote:
> All of the irqfds would to be updated when update the irq
> routing, it's too expensive if there're too many irqfds.
> 
> However we can reduce the cost by avoid some unnecessary
> updates. For irqs of MSI type on X86, the update can be
> saved if the msi values are not change.
> 
> The vfio migration could receives benefit from this optimi-
> zaiton. The test VM has 128 vcpus and 8 VF (with 65 vectors
> enabled), so the VM has more than 520 irqfds. We mesure the
> cost of the vfio_msix_enable (in QEMU, it would set routing
> for each irqfd) for each VF, and we can see the total cost
> can be significantly reduced.
> 
>                  Origin         Apply this Patch
> 1st             8              4
> 2nd             15             5
> 3rd             22             6
> 4th             24             6
> 5th             36             7
> 6th             44             7
> 7th             51             8
> 8th             58             8
> Total           258ms          51ms
> 
> We're also tring to optimize the QEMU part [1], but it's still
> worth to optimize the KVM to gain more benefits.
> 
> [1] https://lists.gnu.org/archive/html/qemu-devel/2021-08/msg04215.html
> 
> Signed-off-by: Longpeng(Mike) <longpeng2@huawei.com>
> ---
>   arch/x86/kvm/x86.c       |  9 +++++++++
>   include/linux/kvm_host.h |  2 ++
>   virt/kvm/eventfd.c       | 15 ++++++++++++++-
>   3 files changed, 25 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index e5d5c5e..22cf20e 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -12023,6 +12023,15 @@ int kvm_arch_update_irqfd_routing(struct kvm *kvm, unsigned int host_irq,
>   	return static_call(kvm_x86_update_pi_irte)(kvm, host_irq, guest_irq, set);
>   }
>   
> +bool kvm_arch_irqfd_route_changed(struct kvm_kernel_irq_routing_entry *old,
> +				  struct kvm_kernel_irq_routing_entry *new)
> +{
> +	if (new->type != KVM_IRQ_ROUTING_MSI)
> +		return true;
> +
> +	return !!memcmp(&old->msi, &new->msi, sizeof(new->msi));
> +}
> +
>   bool kvm_vector_hashing_enabled(void)
>   {
>   	return vector_hashing;
> diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> index ae7735b..c0954ae 100644
> --- a/include/linux/kvm_host.h
> +++ b/include/linux/kvm_host.h
> @@ -1621,6 +1621,8 @@ void kvm_arch_irq_bypass_del_producer(struct irq_bypass_consumer *,
>   void kvm_arch_irq_bypass_start(struct irq_bypass_consumer *);
>   int kvm_arch_update_irqfd_routing(struct kvm *kvm, unsigned int host_irq,
>   				  uint32_t guest_irq, bool set);
> +bool kvm_arch_irqfd_route_changed(struct kvm_kernel_irq_routing_entry *,
> +				  struct kvm_kernel_irq_routing_entry *);
>   #endif /* CONFIG_HAVE_KVM_IRQ_BYPASS */
>   
>   #ifdef CONFIG_HAVE_KVM_INVALID_WAKEUPS
> diff --git a/virt/kvm/eventfd.c b/virt/kvm/eventfd.c
> index e996989..2ad013b 100644
> --- a/virt/kvm/eventfd.c
> +++ b/virt/kvm/eventfd.c
> @@ -281,6 +281,13 @@ int  __attribute__((weak)) kvm_arch_update_irqfd_routing(
>   {
>   	return 0;
>   }
> +
> +bool __attribute__((weak)) kvm_arch_irqfd_route_changed(
> +				struct kvm_kernel_irq_routing_entry *old,
> +				struct kvm_kernel_irq_routing_entry *new)
> +{
> +	return true;
> +}
>   #endif
>   
>   static int
> @@ -615,10 +622,16 @@ void kvm_irq_routing_update(struct kvm *kvm)
>   	spin_lock_irq(&kvm->irqfds.lock);
>   
>   	list_for_each_entry(irqfd, &kvm->irqfds.items, list) {
> +#ifdef CONFIG_HAVE_KVM_IRQ_BYPASS
> +		/* Under irqfds.lock, so can read irq_entry safely */
> +		struct kvm_kernel_irq_routing_entry old = irqfd->irq_entry;
> +#endif
> +
>   		irqfd_update(kvm, irqfd);
>   
>   #ifdef CONFIG_HAVE_KVM_IRQ_BYPASS
> -		if (irqfd->producer) {
> +		if (irqfd->producer &&
> +		    kvm_arch_irqfd_route_changed(&old, &irqfd->irq_entry)) {
>   			int ret = kvm_arch_update_irqfd_routing(
>   					irqfd->kvm, irqfd->producer->irq,
>   					irqfd->gsi, 1);
> 

Queued, thanks.

Paolo