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

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Date:   Thu, 24 Nov 2022 11:27:20 +0100
From:   Janosch Frank <frankja@...ux.ibm.com>
To:     Thomas Huth <thuth@...hat.com>, kvm@...r.kernel.org,
        Claudio Imbrenda <imbrenda@...ux.ibm.com>
Cc:     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 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@...hat.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.