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

[Pierre Morel]

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