Re: [PATCH 4/6] s390: add options to change branch prediction behaviour for the kernel

[Pavel Machek <pavel@ucw.cz>]

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On Wed 2018-02-07 08:00:09, Martin Schwidefsky 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: Cornelia Huck <cohuck@redhat.com>
> Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>
> ---
>  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          | 48 +++++++++++++++++++++++++++++++++++++++
>  arch/s390/kernel/ipl.c            |  1 +
>  arch/s390/kernel/smp.c            |  2 ++
>  7 files changed, 94 insertions(+)
> 
> diff --git a/arch/s390/Kconfig b/arch/s390/Kconfig
> index 0105ce2..d514e25 100644
> --- a/arch/s390/Kconfig
> +++ b/arch/s390/Kconfig
> @@ -540,6 +540,23 @@ config ARCH_RANDOM
>  
>  	  If unsure, say Y.
>  
> +config KERNEL_NOBP
> +	def_bool n
> +	prompt "Enable modified branch prediction for the kernel by default"
> +	help
> +	  If this option is selected the kernel will switch to a modified
> +	  branch prediction mode if the firmware interface is available.
> +	  The modified branch prediction mode improves the behaviour in
> +	  regard to speculative execution.
> +
> +	  With the option enabled the kernel parameter "nobp=0" or "nospec"
> +	  can be used to run the kernel in the normal branch prediction mode.
> +
> +	  With the option disabled the modified branch prediction mode is
> +	  enabled with the "nobp=1" kernel parameter.
> +
> +	  If unsure, say N.
> +

This could use some improvement.

Afaict the config option only changes the default behaviour? Do we
need the option in such case? (CONFIG_CMDLINE_APPEND can be useful to
avoid some options).

Plus, it is not clear what "modified branch prediction mode is" and if
"improves behaviour" means "faster" or "safer".

									Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html

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