Re: [PATCH v4 3/3] KVM: s390: Add opportunistic floating IRQ injection

[Christian Borntraeger <borntraeger@linux.ibm.com>]

Am 20.08.26 um 14:12 schrieb Janosch Frank:
> Try to distribute floating IRQs which haven't been taken by a VCPU yet
> to sleeping VCPUs. This lowers the risk of floating IRQs not being
> delivered.

I think this could be too expensive in the innermost loop.
As an alternative:
what about actually adding a check in handle wait that triggers if we are going
to sleep despite pending interrupts.> 
> Signed-off-by: Janosch Frank <frankja@linux.ibm.com>
> ---
>   arch/s390/include/asm/kvm_host.h |  1 +
>   arch/s390/kvm/interrupt.c        | 31 +++++++++++++++++++++++++++++++
>   arch/s390/kvm/kvm-s390.c         |  3 +++
>   arch/s390/kvm/kvm-s390.h         |  1 +
>   4 files changed, 36 insertions(+)
> 
> diff --git a/arch/s390/include/asm/kvm_host.h b/arch/s390/include/asm/kvm_host.h
> index b4182ca4435f..1d62bdd7aca5 100644
> --- a/arch/s390/include/asm/kvm_host.h
> +++ b/arch/s390/include/asm/kvm_host.h
> @@ -467,6 +467,7 @@ struct kvm_vm_stat {
>   	u64 gmap_shadow_r3_entry;
>   	u64 gmap_shadow_sg_entry;
>   	u64 gmap_shadow_pg_entry;
> +	u64 inject_redist;
>   };
>   
>   struct kvm_arch_memory_slot {
> diff --git a/arch/s390/kvm/interrupt.c b/arch/s390/kvm/interrupt.c
> index b3e4bfeacff2..54f20347f553 100644
> --- a/arch/s390/kvm/interrupt.c
> +++ b/arch/s390/kvm/interrupt.c
> @@ -367,6 +367,37 @@ static unsigned long deliverable_irqs(struct kvm_vcpu *vcpu)
>   	return active_mask;
>   }
>   
> +void distribute_float_irqs(struct kvm *kvm)
> +{
> +	struct kvm_vcpu *dst_vcpu;
> +	int sigcpu, online_vcpus;
> +
> +	if (!READ_ONCE(kvm->arch.float_int.pending_irqs))
> +		return;
> +
> +	online_vcpus = atomic_read(&kvm->online_vcpus);
> +
> +	/*
> +	 * Not too worried about synchronization for idle_mask. We
> +	 * might burn too many cycles but apart from that waking a
> +	 * vcpu is not harmful.
> +	 */
> +	sigcpu = find_first_bit(kvm->arch.idle_mask, online_vcpus);
> +	/* Well nobody's sleeping so someone will likely take the IRQ soon */
> +	if (sigcpu == online_vcpus)
> +		return;
> +
> +	do {
> +		dst_vcpu = kvm_get_vcpu(kvm, sigcpu);
> +		if (deliverable_irqs(dst_vcpu)) {
> +			kvm->stat.inject_redist++;
> +			kvm_s390_vcpu_wakeup(dst_vcpu);
> +			break;
> +		}
> +		sigcpu = find_next_bit(kvm->arch.idle_mask, online_vcpus, ++sigcpu);
> +	} while (sigcpu < online_vcpus);
> +}
> +
>   static void __set_cpu_idle(struct kvm_vcpu *vcpu)
>   {
>   	kvm_s390_set_cpuflags(vcpu, CPUSTAT_WAIT);
> diff --git a/arch/s390/kvm/kvm-s390.c b/arch/s390/kvm/kvm-s390.c
> index 518a69c55e85..d6d46688d980 100644
> --- a/arch/s390/kvm/kvm-s390.c
> +++ b/arch/s390/kvm/kvm-s390.c
> @@ -86,6 +86,7 @@ const struct kvm_stats_desc kvm_vm_stats_desc[] = {
>   	STATS_DESC_COUNTER(VM, gmap_shadow_r3_entry),
>   	STATS_DESC_COUNTER(VM, gmap_shadow_sg_entry),
>   	STATS_DESC_COUNTER(VM, gmap_shadow_pg_entry),
> +	STATS_DESC_COUNTER(VM, inject_redist),
>   };
>   
>   const struct kvm_stats_header kvm_vm_stats_header = {
> @@ -4572,6 +4573,8 @@ static int vcpu_pre_run(struct kvm_vcpu *vcpu)
>   		rc = kvm_s390_deliver_pending_interrupts(vcpu);
>   		if (rc || guestdbg_exit_pending(vcpu))
>   			return rc;
> +
> +		distribute_float_irqs(vcpu->kvm);
>   	}
>   
>   	rc = kvm_s390_handle_requests(vcpu);
> diff --git a/arch/s390/kvm/kvm-s390.h b/arch/s390/kvm/kvm-s390.h
> index 8e886bcef4a0..7115a8f5b188 100644
> --- a/arch/s390/kvm/kvm-s390.h
> +++ b/arch/s390/kvm/kvm-s390.h
> @@ -376,6 +376,7 @@ int __must_check kvm_s390_deliver_pending_interrupts(struct kvm_vcpu *vcpu);
>   void kvm_s390_clear_local_irqs(struct kvm_vcpu *vcpu);
>   void kvm_s390_clear_float_irqs(struct kvm *kvm);
>   void kvm_s390_pv_sclp_kick(struct kvm_vcpu *vcpu);
> +void distribute_float_irqs(struct kvm *kvm);
>   int __must_check kvm_s390_inject_vm(struct kvm *kvm,
>   				    struct kvm_s390_interrupt *s390int,
>   				    struct kvm_s390_interrupt_info *inti);