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

Heiko Carstens
2021-12-13T15:21:58Z

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.