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

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On 20.02.2018 16:36, Christian Borntraeger wrote:
> 
> 
> On 02/07/2018 12:46 PM, 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@xxxxxxxxxx>
>> ---
>>  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;
> 
> we only add it to epdx, so should it be u8?

Indeed, this should be u8.

>> +
>> +	/*
>> +	 * 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;
> 
> 		and -1 then here?

Yes, thanks!

> 
>> +
>> +	scb->epoch += delta;
>> +	if (scb->ecd & ECD_MEF) {
>> +		scb->epdx += delta_idx;
>> +		if (scb->epoch < delta)
>> +			scb->epdx += 1;
> 
> 


-- 

Thanks,

David / dhildenb



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