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

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Date:   Mon, 13 Dec 2021 16:46:07 +0100
From:   Pierre Morel <pmorel@...ux.ibm.com>
To:     Heiko Carstens <hca@...ux.ibm.com>
Cc:     Claudio Imbrenda <imbrenda@...ux.ibm.com>, 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 12/13/21 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