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

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

[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")
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))
-- 
2.31.1

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

[David Hildenbrand]

On 23.11.22 10:08, Thomas Huth wrote:
> 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")
> 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))

Nice,

Acked-by: David Hildenbrand <david@redhat.com>

-- 
Thanks,

David / dhildenb

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

[Claudio Imbrenda]

On Wed, 23 Nov 2022 10:08:33 +0100
Thomas Huth <thuth@redhat.com> wrote:

> 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")
> Signed-off-by: Thomas Huth <thuth@redhat.com>

Reviewed-by: Claudio Imbrenda <imbrenda@linux.ibm.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;

looks quite straightforward

> +	}
>  
>  	/* etoken */
>  	if (test_kvm_facility(vcpu->kvm, 156))

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

[Christian Borntraeger]

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.

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))

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

[Janosch Frank]

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))

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

[Janosch Frank]

On 11/23/22 10:08, Thomas Huth wrote:
> 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")
> Signed-off-by: Thomas Huth <thuth@redhat.com>

Could you please add a test for this to the KVM unit tests?
I'd guess you might already have some code for it from your debugging 
sessions.

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

[Thomas Huth]

On 24/11/2022 11.27, Janosch Frank wrote:
> On 11/23/22 10:08, Thomas Huth wrote:
>> 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")
>> Signed-off-by: Thomas Huth <thuth@redhat.com>
> 
> Could you please add a test for this to the KVM unit tests?
> I'd guess you might already have some code for it from your debugging sessions.

I don't have some test code for this yet - I was only testing with the 
scenario that is described in the bugzilla ticket. But sure, I can have a 
try to come up with a k-u-t test.

  Thomas