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

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Date:   Mon, 13 Dec 2021 16:21:48 +0100
From:   Heiko Carstens <hca@...ux.ibm.com>
To:     Pierre Morel <pmorel@...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 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.