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Date: Mon, 23 Jul 2018 17:43:06 +0530
From: Gautham R Shenoy <ego@linux.vnet.ibm.com>
To: Greg KH <gregkh@linuxfoundation.org>
Cc: "Gautham R. Shenoy" <ego@linux.vnet.ibm.com>,
        stable@vger.kernel.org, Michael Ellerman <mpe@ellerman.id.au>
Subject: Re: [PATCH] powerpc/powernv : Save/Restore SPRG3 on entry/exit from
 stop.
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Hello Greg,

On Mon, Jul 23, 2018 at 09:53:50AM +0200, Greg KH wrote:
> On Mon, Jul 23, 2018 at 08:16:28AM +0200, Greg KH wrote:
> > On Mon, Jul 23, 2018 at 11:00:55AM +0530, Gautham R. Shenoy wrote:
> > > From: "Gautham R. Shenoy" <ego@linux.vnet.ibm.com>
> > > 
> > > [ Upstream commit b03897cf318dfc47de33a7ecbc7655584266f034 ]
> > > 
> > > On 64-bit servers, SPRN_SPRG3 and its userspace read-only mirror
> > > SPRN_USPRG3 are used as userspace VDSO write and read registers
> > > respectively.
> > > 
> > > SPRN_SPRG3 is lost when we enter stop4 and above, and is currently not
> > > restored.  As a result, any read from SPRN_USPRG3 returns zero on an
> > > exit from stop4 and above.
> > > 
> > > Thus in this situation, on POWER9, any call from sched_getcpu() always
> > > returns zero, as on powerpc, we call __kernel_getcpu() which relies
> > > upon SPRN_USPRG3 to report the CPU and NUMA node information.
> > > 
> > > Fix this by restoring SPRN_SPRG3 on wake up from a deep stop state
> > > with the sprg_vdso value that is cached in PACA.
> > > 
> > > Fixes: e1c1cfed5432 ("powerpc/powernv: Save/Restore additional SPRs
> > > for stop4 cpuidle")
> > > 
> > > Reported-by: Florian Weimer <fweimer@redhat.com>
> > > Cc: <stable@vger.kernel.org> # 4.14
> > > Signed-off-by: Gautham R. Shenoy <ego@linux.vnet.ibm.com>
> > > Reviewed-by: Michael Ellerman <mpe@ellerman.id.au>
> > > Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
> > > ---
> > >  arch/powerpc/kernel/idle_book3s.S | 2 ++
> > >  1 file changed, 2 insertions(+)
> > 
> > What stable tree(s) do you want this applied to?
> 
> I've now guessed that it is 4.14.y.  If that's not the case, please let
> me know.

Yes, this backport is for 4.14.y. The upstream commit cleanly applies
on 4.15.18 and 4.16.18.

> 
> greg k-h
> 

--
Thanks and Regards
gautham.