linux-kernel - Re: [PATCH v5 1/1] s390x: KVM: accept STSI for CPU topology information

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Date:   Mon, 13 Dec 2021 15:26:58 +0100
From:   Pierre Morel <pmorel@...ux.ibm.com>
To:     Heiko Carstens <hca@...ux.ibm.com>,
        Claudio Imbrenda <imbrenda@...ux.ibm.com>
Cc:     kvm@...r.kernel.org, linux-s390@...r.kernel.org,
        linux-kernel@...r.kernel.org, borntraeger@...ibm.com,
        frankja@...ux.ibm.com, cohuck@...hat.com, david@...hat.com,
        thuth@...hat.com, gor@...ux.ibm.com
Subject: Re: [PATCH v5 1/1] s390x: KVM: accept STSI for CPU topology
 information



On 12/9/21 16:54, Heiko Carstens wrote:
> On Thu, Dec 09, 2021 at 01:36:16PM +0100, Claudio Imbrenda wrote:
>> On Mon, 22 Nov 2021 14:14:43 +0100
>> Pierre Morel <pmorel@...ux.ibm.com> wrote:
>>
>>> We let the userland hypervisor know if the machine support the CPU
>>> topology facility using a new KVM capability: KVM_CAP_S390_CPU_TOPOLOGY.
>>>
>>> The PTF instruction will report a topology change if there is any change
>>> with a previous STSI_15_1_2 SYSIB.
>>> Changes inside a STSI_15_1_2 SYSIB occur if CPU bits are set or clear
>>> inside the CPU Topology List Entry CPU mask field, which happens with
>>> changes in CPU polarization, dedication, CPU types and adding or
>>> removing CPUs in a socket.
>>>
>>> The reporting to the guest is done using the Multiprocessor
>>> Topology-Change-Report (MTCR) bit of the utility entry of the guest's
>>> SCA which will be cleared during the interpretation of PTF.
>>>
>>> To check if the topology has been modified we use a new field of the
>>> arch vCPU to save the previous real CPU ID at the end of a schedule
>>> and verify on next schedule that the CPU used is in the same socket.
>>>
>>> We assume in this patch:
>>> - no polarization change: only horizontal polarization is currently
>>>    used in linux.
> 
> Why is this assumption necessary? The statement that Linux runs only
> with horizontal polarization is not true.
> 

Right, I will rephrase this as:

"Polarization change is not taken into account, QEMU intercepts queries 
for polarization change (PTF) and only provides horizontal polarization 
indication to Guest's Linux."

@Heiko, I did not find any usage of the polarization in the kernel other 
than an indication in the sysfs. Is there currently other use of the 
polarization that I did not see?



-- 
Pierre Morel
IBM Lab Boeblingen