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

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Message-ID: <ef6d7ca6-e896-b554-6a30-05113114713d@redhat.com>
Date:   Thu, 24 Nov 2022 11:46:38 +0100
From:   Thomas Huth <thuth@...hat.com>
To:     Janosch Frank <frankja@...ux.ibm.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 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@...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.

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