[PATCH stable v4.14 13/32] powerpc/fsl: Add barrier_nospec implementation for NXP PowerPC Book3E

[Joakim Tjernlund]

On Tue, 2019-04-02 at 17:19 +1100, Michael Ellerman wrote:
> 
> Joakim Tjernlund <Joakim.Tjernlund at infinera.com> writes:
> > On Fri, 2019-03-29 at 22:26 +1100, Michael Ellerman wrote:
> > > From: Diana Craciun <diana.craciun at nxp.com>
> > > 
> > > commit ebcd1bfc33c7a90df941df68a6e5d4018c022fba upstream.
> > > 
> > > Implement the barrier_nospec as a isync;sync instruction sequence.
> > > The implementation uses the infrastructure built for BOOK3S 64.
> > > 
> > > Signed-off-by: Diana Craciun <diana.craciun at nxp.com>
> > > [mpe: Split out of larger patch]
> > > Signed-off-by: Michael Ellerman <mpe at ellerman.id.au>
> > 
> > What is the performanc impact of these spectre fixes?
> 
> I've not seen any numbers from anyone.

Thanks for getting back to me.

> 
> It will depend on the workload, it's copy to/from user that is most
> likely to show an impact.
> 
> We have a context switch benchmark in
> tools/testing/selftests/powerpc/benchmarks/context_switch.c.
> 
> Running that with "--no-vector --no-altivec --no-fp --test=pipe" shows
> about a 2.3% slow down vs booting with "nospectre_v1".
> 
> > Can I compile it away?
> 
> You can't actually, but you can disable it at runtime with
> "nospectre_v1" on the kernel command line.
> 
> We could make it a user selectable compile time option if you really
> want it to be.

I think yes. Considering that these patches are fairly untested and the impact
in the wild unknown. Requiring systems to change their boot config over night is
too fast.

 Jocke