Re: [PATCH 1/1] KVM: s390: diag9c forwarding

[Cornelia Huck <cohuck@redhat.com>]

On Mon, 18 Jan 2021 14:17:39 +0100
Christian Borntraeger <borntraeger@de.ibm.com> wrote:

> From: Pierre Morel <pmorel@linux.ibm.com>
> 
> When we receive intercept a DIAG_9C from the guest we verify
> that the target real CPU associated with the virtual CPU
> designated by the guest is running and if not we forward the
> DIAG_9C to the target real CPU.
> 
> To avoid a diag9c storm we allow a maximal rate of diag9c forwarding.
> 
> The rate is calculated as a count per second defined as a
> new parameter of the s390 kvm module: diag9c_forwarding_hz .
> 
> The default value is to not forward diag9c.
> 
> Signed-off-by: Pierre Morel <pmorel@linux.ibm.com>
> Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
> ---
>  arch/s390/include/asm/kvm_host.h |  1 +
>  arch/s390/include/asm/smp.h      |  1 +
>  arch/s390/kernel/smp.c           |  1 +
>  arch/s390/kvm/diag.c             | 31 ++++++++++++++++++++++++++++---
>  arch/s390/kvm/kvm-s390.c         |  6 ++++++
>  arch/s390/kvm/kvm-s390.h         |  8 ++++++++
>  6 files changed, 45 insertions(+), 3 deletions(-)
> 

(...)

> @@ -167,9 +180,21 @@ static int __diag_time_slice_end_directed(struct kvm_vcpu *vcpu)
>  	if (!tcpu)
>  		goto no_yield;
>  
> -	/* target already running */
> -	if (READ_ONCE(tcpu->cpu) >= 0)
> -		goto no_yield;
> +	/* target VCPU already running */

Maybe make this /* target guest VPCU already running */...

> +	if (READ_ONCE(tcpu->cpu) >= 0) {
> +		if (!diag9c_forwarding_hz || diag9c_forwarding_overrun())
> +			goto no_yield;
> +
> +		/* target CPU already running */

...and this /* target host CPU already running */? I just read this
several times and was confused before I spotted the difference :)

> +		if (!vcpu_is_preempted(tcpu->cpu))
> +			goto no_yield;
> +		smp_yield_cpu(tcpu->cpu);
> +		VCPU_EVENT(vcpu, 5,
> +			   "diag time slice end directed to %d: yield forwarded",
> +			   tid);
> +		vcpu->stat.diagnose_9c_forward++;
> +		return 0;
> +	}
>  
>  	if (kvm_vcpu_yield_to(tcpu) <= 0)
>  		goto no_yield;

(...)