Message-ID: <1b2903fa-7b83-418d-8fa6-9bdf9ad19640@linux.ibm.com>
Date:   Mon, 13 Dec 2021 16:46:07 +0100
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Subject: Re: [PATCH v5 1/1] s390x: KVM: accept STSI for CPU topology
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To:     Heiko Carstens <hca@linux.ibm.com>
Cc:     Claudio Imbrenda <imbrenda@linux.ibm.com>, kvm@vger.kernel.org,
        linux-s390@vger.kernel.org, linux-kernel@vger.kernel.org,
        borntraeger@de.ibm.com, frankja@linux.ibm.com, cohuck@redhat.com,
        david@redhat.com, thuth@redhat.com, gor@linux.ibm.com
References: <20211122131443.66632-1-pmorel@linux.ibm.com>
 <20211122131443.66632-2-pmorel@linux.ibm.com>
 <20211209133616.650491fd@p-imbrenda> <YbImqX/NEus71tZ1@osiris>
 <fbc46b35-10af-2c7e-6e47-e4987070ad83@linux.ibm.com>
 <YbdlDFLjZzpC6RRd@osiris>
From:   Pierre Morel <pmorel@linux.ibm.com>
In-Reply-To: <YbdlDFLjZzpC6RRd@osiris>
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On 12/13/21 16:21, Heiko Carstens wrote:
> On Mon, Dec 13, 2021 at 03:26:58PM +0100, Pierre Morel wrote:
>>> 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?
> 
> You can change polarization by writing to /sys/devices/system/cpu/dispatching.
> 
> Or alternativel use the chcpu tool to change polarization. There is
> however no real support for vertical polarization implemented in the
> kernel. Therefore changing to vertical polarization is _not_
> recommended, since it will most likely have negative performance
> impacts on your Linux system.
> However the interface is still there for experimental purposes.
> 

Thanks, so I guess that not reflecting polarization changes to the guest 
topology will be OK for the moment.
Of course, I will change the wrong comment.

-- 
Pierre Morel
IBM Lab Boeblingen