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

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Message-ID: <20221123103859.1d861c44@p-imbrenda>
Date:   Wed, 23 Nov 2022 10:38:59 +0100
From:   Claudio Imbrenda <imbrenda@...ux.ibm.com>
To:     Thomas Huth <thuth@...hat.com>
Cc:     kvm@...r.kernel.org, Janosch Frank <frankja@...ux.ibm.com>,
        linux-s390@...r.kernel.org, linux-kernel@...r.kernel.org,
        Christian Borntraeger <borntraeger@...ux.ibm.com>,
        David Hildenbrand <david@...hat.com>,
        "Collin L. Walling" <walling@...ux.ibm.com>,
        Jason J Herne <jjherne@...ux.ibm.com>
Subject: Re: [PATCH] KVM: s390: vsie: Fix the initialization of the epoch
 extension (epdx) field

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

Reviewed-by: Claudio Imbrenda <imbrenda@...ux.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))