Re: [PATCH 3/6] s390: add options to change branch prediction behaviour for the kernel

[Cornelia Huck]

On Wed, 17 Jan 2018 10:48:36 +0100
Martin Schwidefsky <schwidefsky@xxxxxxxxxx> wrote:

> Add the PPA instruction to the system entry and exit path to switch
> the kernel to a different branch prediction behaviour. The instructions
> are added via CPU alternatives and can be disabled with the "nospec"
> or the "nobp=0" kernel parameter. If the default behaviour selected
> with CONFIG_KERNEL_NOBP is set to "n" then the "nobp=1" parameter can be
> used to enable the changed kernel branch prediction.
> 
> Acked-by: Christian Borntraeger <borntraeger@xxxxxxxxxx>
> Signed-off-by: Martin Schwidefsky <schwidefsky@xxxxxxxxxx>
> ---
>  arch/s390/Kconfig                 | 17 +++++++++++++
>  arch/s390/include/asm/processor.h |  1 +
>  arch/s390/kernel/alternative.c    | 23 ++++++++++++++++++
>  arch/s390/kernel/early.c          |  2 ++
>  arch/s390/kernel/entry.S          | 50 ++++++++++++++++++++++++++++++++++++++-
>  arch/s390/kernel/ipl.c            |  1 +
>  arch/s390/kernel/smp.c            |  2 ++
>  7 files changed, 95 insertions(+), 1 deletion(-)

Acked-by: Cornelia Huck <cohuck@xxxxxxxxxx>

[This looks sane; but as I don't have insight into the details of the
new instructions, I don't feel confident enough to give a R-b]