Re: [PATCH RFC 4/6] KVM: s390: consider epoch index on TOD clock syncs

["Collin L. Walling" <walling@linux.vnet.ibm.com>]

On 02/07/2018 06:46 AM, David Hildenbrand wrote:
> For now, we don't take care of over/underflows. Especially underflows
> are critical:
>
> Assume the epoch is currently 0 and we get a sync request for delta=1,
> meaning the TOD is moved forward by 1 and we have to fix it up by
> subtracting 1 from the epoch. Right now, this will leave the epoch
> index untouched, resulting in epoch=-1, epoch_idx=0, which is wrong.
>
> We have to take care of over and underflows, also for the VSIE case. So
> let's factor out calculation into a separate function.
>
> Signed-off-by: David Hildenbrand <david@redhat.com>
> ---
>   arch/s390/kvm/kvm-s390.c | 32 +++++++++++++++++++++++++++++---
>   1 file changed, 29 insertions(+), 3 deletions(-)
>
> diff --git a/arch/s390/kvm/kvm-s390.c b/arch/s390/kvm/kvm-s390.c
> index d007b737cd4d..c2b62379049e 100644
> --- a/arch/s390/kvm/kvm-s390.c
> +++ b/arch/s390/kvm/kvm-s390.c
> @@ -179,6 +179,28 @@ int kvm_arch_hardware_enable(void)
>   static void kvm_gmap_notifier(struct gmap *gmap, unsigned long start,
>   			      unsigned long end);
>
> +static void kvm_clock_sync_scb(struct kvm_s390_sie_block *scb, u64 delta)
> +{
> +	u64 delta_idx = 0;
> +
> +	/*
> +	 * The TOD jumps by delta, we have to compensate this by adding
> +	 * -delta to the epoch.
> +	 */
> +	delta = -delta;
> +
> +	/* sign-extension - we're adding to signed values below */
> +	if ((s64)delta < 0)
> +		delta_idx = 0xff;
> +
> +	scb->epoch += delta;
> +	if (scb->ecd & ECD_MEF) {
> +		scb->epdx += delta_idx;
> +		if (scb->epoch < delta)
> +			scb->epdx += 1;
> +	}
> +}
> +

Is the sync always a jump forward? Do we need to worry about a borrow 
from the epdx in case of underflow?

>   /*
>    * This callback is executed during stop_machine(). All CPUs are therefore
>    * temporarily stopped. In order not to change guest behavior, we have to
> @@ -194,13 +216,17 @@ static int kvm_clock_sync(struct notifier_block *notifier, unsigned long val,
>   	unsigned long long *delta = v;
>
>   	list_for_each_entry(kvm, &vm_list, vm_list) {
> -		kvm->arch.epoch -= *delta;
>   		kvm_for_each_vcpu(i, vcpu, kvm) {
> -			vcpu->arch.sie_block->epoch -= *delta;
> +			kvm_clock_sync_scb(vcpu->arch.sie_block, *delta);
> +			if (i == 0) {
> +				kvm->arch.epoch = vcpu->arch.sie_block->epoch;
> +				kvm->arch.epdx = vcpu->arch.sie_block->epdx;

Are we safe by setting the kvm epochs to the sie epochs wrt migration?

> +			}
>   			if (vcpu->arch.cputm_enabled)
>   				vcpu->arch.cputm_start += *delta;
>   			if (vcpu->arch.vsie_block)
> -				vcpu->arch.vsie_block->epoch -= *delta;
> +				kvm_clock_sync_scb(vcpu->arch.vsie_block,
> +						   *delta);
>   		}
>   	}
>   	return NOTIFY_OK;

-- 
- Collin L Walling