Re: [PATCH v13 0/7] s390x: CPU Topology

From: Thomas Huth <thuth@redhat.com>
To: Pierre Morel <pmorel@linux.ibm.com>, qemu-s390x@nongnu.org
Cc: qemu-devel@nongnu.org, borntraeger@de.ibm.com,
	pasic@linux.ibm.com, richard.henderson@linaro.org,
	david@redhat.com, cohuck@redhat.com, mst@redhat.com,
	pbonzini@redhat.com, kvm@vger.kernel.org, ehabkost@redhat.com,
	marcel.apfelbaum@gmail.com, eblake@redhat.com, armbru@redhat.com,
	seiden@linux.ibm.com, nrb@linux.ibm.com, scgl@linux.ibm.com,
	frankja@linux.ibm.com, berrange@redhat.com, clg@kaod.org
Subject: Re: [PATCH v13 0/7] s390x: CPU Topology
Date: Mon, 12 Dec 2022 11:17:21 +0100	[thread overview]
Message-ID: <73238c6c-a9dc-9d18-8ffb-92c8a41922d3@redhat.com> (raw)
In-Reply-To: <90514038-f10c-33e7-3600-e3131138a44d@linux.ibm.com>

On 12/12/2022 11.10, Pierre Morel wrote:
> 
> 
> On 12/12/22 10:07, Thomas Huth wrote:
>> On 12/12/2022 09.51, Pierre Morel wrote:
>>>
>>>
>>> On 12/9/22 14:32, Thomas Huth wrote:
>>>> On 08/12/2022 10.44, Pierre Morel wrote:
>>>>> Hi,
>>>>>
>>>>> Implementation discussions
>>>>> ==========================
>>>>>
>>>>> CPU models
>>>>> ----------
>>>>>
>>>>> Since the S390_FEAT_CONFIGURATION_TOPOLOGY is already in the CPU model
>>>>> for old QEMU we could not activate it as usual from KVM but needed
>>>>> a KVM capability: KVM_CAP_S390_CPU_TOPOLOGY.
>>>>> Checking and enabling this capability enables
>>>>> S390_FEAT_CONFIGURATION_TOPOLOGY.
>>>>>
>>>>> Migration
>>>>> ---------
>>>>>
>>>>> Once the S390_FEAT_CONFIGURATION_TOPOLOGY is enabled in the source
>>>>> host the STFL(11) is provided to the guest.
>>>>> Since the feature is already in the CPU model of older QEMU,
>>>>> a migration from a new QEMU enabling the topology to an old QEMU
>>>>> will keep STFL(11) enabled making the guest get an exception for
>>>>> illegal operation as soon as it uses the PTF instruction.
>>>>
>>>> I now thought that it is not possible to enable "ctop" on older QEMUs 
>>>> since the don't enable the KVM capability? ... or is it still somehow 
>>>> possible? What did I miss?
>>>>
>>>>   Thomas
>>>
>>> Enabling ctop with ctop=on on old QEMU is not possible, this is right.
>>> But, if STFL(11) is enable in the source KVM by a new QEMU, I can see 
>>> that even with -ctop=off the STFL(11) is migrated to the destination.
>>
>> Is this with the "host" CPU model or another one? And did you explicitly 
>> specify "ctop=off" at the command line, or are you just using the default 
>> setting by not specifying it?
> 
> With explicit cpumodel and using ctop=off like in
> 
> sudo /usr/local/bin/qemu-system-s390x_master \
>       -m 512M \
>       -enable-kvm -smp 4,sockets=4,cores=2,maxcpus=8 \
>       -cpu z14,ctop=off \
>       -machine s390-ccw-virtio-7.2,accel=kvm \
>       ...

Ok ... that sounds like a bug somewhere in your patches or in the kernel 
code ... the guest should never see STFL bit 11 if ctop=off, should it?

  Thomas