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

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Date:   Thu, 18 Jan 2018 10:52:23 +0100
From:   Cornelia Huck <cohuck@...hat.com>
To:     Martin Schwidefsky <schwidefsky@...ibm.com>
Cc:     linux-kernel@...r.kernel.org, linux-s390@...r.kernel.org,
        kvm@...r.kernel.org, Heiko Carstens <heiko.carstens@...ibm.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jon Masters <jcm@...hat.com>,
        Marcus Meissner <meissner@...e.de>,
        Jiri Kosina <jkosina@...e.cz>
Subject: Re: [PATCH 3/6] s390: add options to change branch prediction
 behaviour for the kernel

On Wed, 17 Jan 2018 10:48:36 +0100
Martin Schwidefsky <schwidefsky@...ibm.com> 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@...ibm.com>
> Signed-off-by: Martin Schwidefsky <schwidefsky@...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          | 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@...hat.com>

[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]