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@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 | 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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