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

[Michael Ellerman]

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.

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.

cheers