Re: [PATCH stable v4.8] KVM: PPC: Book3S: Treat VTB as a per-subcore register, not per-thread

[Greg KH <gregkh@linuxfoundation.org>]

On Wed, Mar 08, 2017 at 11:41:32AM +1100, Paul Mackerras wrote:
> commit 88b02cf97bb7e742db3e31671d54177e3e19fd89 upstream.
> 
> POWER8 has one virtual timebase (VTB) register per subcore, not one
> per CPU thread.  The HV KVM code currently treats VTB as a per-thread
> register, which can lead to spurious soft lockup messages from guests
> which use the VTB as the time source for the soft lockup detector.
> (CPUs before POWER8 did not have the VTB register.)
> 
> For HV KVM, this fixes the problem by making only the primary thread
> in each virtual core save and restore the VTB value.  With this,
> the VTB state becomes part of the kvmppc_vcore structure.  This
> also means that "piggybacking" of multiple virtual cores onto one
> subcore is not possible on POWER8, because then the virtual cores
> would share a single VTB register.
> 
> PR KVM emulates a VTB register, which is per-vcpu because PR KVM
> has no notion of CPU threads or SMT.  For PR KVM we move the VTB
> state into the kvmppc_vcpu_book3s struct.
> 
> Cc: stable@vger.kernel.org # v3.14+
> Reported-by: Thomas Huth <thuth@redhat.com>
> Tested-by: Thomas Huth <thuth@redhat.com>
> Signed-off-by: Paul Mackerras <paulus@ozlabs.org>
> ---
> This is intended for the 4.8 stable tree.

4.8 is long gone and not being maintained anymore, so there's not much
we can do with this, sorry.  Was there a specific distro that you were
concerned about not picking this up?

thanks,

greg k-h