Re: [PATCH 1/2] KVM: s390: optimize wakeup for exitless interrupts

[Cornelia Huck <cohuck@redhat.com>]

On Tue, 13 Feb 2018 12:28:57 +0000
Christian Borntraeger <borntraeger@de.ibm.com> wrote:

> For interrupt injection of floating interrupts we queue the interrupt
> either in the GISA or in the floating  interrupt list. The first CPU
> that looks at these data structures - either in KVM code or hardware
> will then deliver that interrupt. To minimize latency we also:
> -a: choose a VCPU to deliver that interrupt. We prefer idle CPUs
> -b: we wake up the host thread that runs the VCPU
> -c: set an I/O intervention bit for that CPU so that it exits guest
>     context as soon as the PSW I/O mask is enabled
> This will make sure that this CPU will execute the interrupt delivery
> code of KVM very soon.
> 
> We can now optimize the injection case if we have exitless interrupts.
> The wakeup is still necessary in case the target CPU sleeps. We can
> avoid the I/O intervention request bit though. Whenever this
> intervention request would be handled, the hardware could also directly
> inject the interrupt on that CPU, no need to go through the interrupt
> injection loop of KVM.
> 
> Cc: Michael Mueller <mimu@linux.vnet.ibm.com>
> Reviewed-by: Halil Pasic <pasic@linux.vnet.ibm.com>
> Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
> ---
>  arch/s390/kvm/interrupt.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/s390/kvm/interrupt.c b/arch/s390/kvm/interrupt.c
> index aabf46f5f883..337a69bc04db 100644
> --- a/arch/s390/kvm/interrupt.c
> +++ b/arch/s390/kvm/interrupt.c
> @@ -1701,7 +1701,8 @@ static void __floating_irq_kick(struct kvm *kvm, u64 type)
>  		kvm_s390_set_cpuflags(dst_vcpu, CPUSTAT_STOP_INT);
>  		break;
>  	case KVM_S390_INT_IO_MIN...KVM_S390_INT_IO_MAX:
> -		kvm_s390_set_cpuflags(dst_vcpu, CPUSTAT_IO_INT);
> +		if (!(type & KVM_S390_INT_IO_AI_MASK && kvm->arch.gisa))
> +			kvm_s390_set_cpuflags(dst_vcpu, CPUSTAT_IO_INT);
>  		break;
>  	default:
>  		kvm_s390_set_cpuflags(dst_vcpu, CPUSTAT_EXT_INT);

Reviewed-by: Cornelia Huck <cohuck@redhat.com>