Date:   Mon, 13 Dec 2021 16:21:48 +0100
From:   Heiko Carstens <hca@linux.ibm.com>
To:     Pierre Morel <pmorel@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
Subject: Re: [PATCH v5 1/1] s390x: KVM: accept STSI for CPU topology
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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.