Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Message-ID: <ef6d7ca6-e896-b554-6a30-05113114713d@redhat.com>
Date:   Thu, 24 Nov 2022 11:46:38 +0100
MIME-Version: 1.0
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101
 Thunderbird/91.13.0
Subject: Re: [PATCH] KVM: s390: vsie: Fix the initialization of the epoch
 extension (epdx) field
Content-Language: en-US
To:     Janosch Frank <frankja@linux.ibm.com>, kvm@vger.kernel.org,
        Claudio Imbrenda <imbrenda@linux.ibm.com>
Cc:     linux-s390@vger.kernel.org, linux-kernel@vger.kernel.org,
        Christian Borntraeger <borntraeger@linux.ibm.com>,
        David Hildenbrand <david@redhat.com>,
        "Collin L. Walling" <walling@linux.ibm.com>,
        Jason J Herne <jjherne@linux.ibm.com>
References: <20221123090833.292938-1-thuth@redhat.com>
 <4d46020f-f33a-474c-b791-be11c2ce2aa6@linux.ibm.com>
From:   Thomas Huth <thuth@redhat.com>
In-Reply-To: <4d46020f-f33a-474c-b791-be11c2ce2aa6@linux.ibm.com>
Content-Type: text/plain; charset=UTF-8; format=flowed
Content-Transfer-Encoding: 7bit
Precedence: bulk

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