Re: [PATCH] KVM: s390: vsie: Fix the initialization of the epoch extension (epdx) field

[Janosch Frank <frankja@linux.ibm.com>]

On 11/23/22 10:41, Christian Borntraeger wrote:
> 
> 
> Am 23.11.22 um 10:08 schrieb Thomas Huth:
>> We recently experienced some weird huge time jumps in nested guests when
>> rebooting them in certain cases. After adding some debug code to the epoch
>> handling in vsie.c (thanks to David Hildenbrand for the idea!), it was
>> obvious that the "epdx" field (the multi-epoch extension) did not get set
>> to 0xff in case the "epoch" field was negative.
>> Seems like the code misses to copy the value from the epdx field from
>> the guest to the shadow control block. By doing so, the weird time
>> jumps are gone in our scenarios.
>>
>> Link: https://bugzilla.redhat.com/show_bug.cgi?id=2140899
>> Fixes: 8fa1696ea781 ("KVM: s390: Multiple Epoch Facility support")
> 
> We might want to add cc stable, just in case.

I'm pushing this to devel for the CI with the following additions:

Reviewed-by: Janosch Frank <frankja@linux.ibm.com>
Cc: stable@vger.kernel.org # 4.19+

> 
> Reviewed-by: Christian Borntraeger <borntraeger@linux.ibm.com>
> 
> 
>> Signed-off-by: Thomas Huth <thuth@redhat.com>
>> ---
>>    arch/s390/kvm/vsie.c | 4 +++-
>>    1 file changed, 3 insertions(+), 1 deletion(-)
>>
>> diff --git a/arch/s390/kvm/vsie.c b/arch/s390/kvm/vsie.c
>> index 94138f8f0c1c..ace2541ababd 100644
>> --- a/arch/s390/kvm/vsie.c
>> +++ b/arch/s390/kvm/vsie.c
>> @@ -546,8 +546,10 @@ static int shadow_scb(struct kvm_vcpu *vcpu, struct vsie_page *vsie_page)
>>    	if (test_kvm_cpu_feat(vcpu->kvm, KVM_S390_VM_CPU_FEAT_CEI))
>>    		scb_s->eca |= scb_o->eca & ECA_CEI;
>>    	/* Epoch Extension */
>> -	if (test_kvm_facility(vcpu->kvm, 139))
>> +	if (test_kvm_facility(vcpu->kvm, 139)) {
>>    		scb_s->ecd |= scb_o->ecd & ECD_MEF;
>> +		scb_s->epdx = scb_o->epdx;
>> +	}
>>    
>>    	/* etoken */
>>    	if (test_kvm_facility(vcpu->kvm, 156))