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Date:   Wed, 6 Jul 2022 10:40:37 +0200
From:   Heiko Carstens <hca@linux.ibm.com>
To:     "Jason A. Donenfeld" <Jason@zx2c4.com>
Cc:     Borislav Petkov <bp@alien8.de>, "H. Peter Anvin" <hpa@zytor.com>,
        linux-arm-kernel@lists.infradead.org,
        linuxppc-dev@lists.ozlabs.org, linux-s390@vger.kernel.org,
        linux-kernel@vger.kernel.org, x86@kernel.org,
        Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will@kernel.org>,
        Michael Ellerman <mpe@ellerman.id.au>,
        Alexander Gordeev <agordeev@linux.ibm.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Arnd Bergmann <arnd@arndb.de>
Subject: Re: [PATCH v2] random: remove CONFIG_ARCH_RANDOM
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 <20220706003225.335768-1-Jason@zx2c4.com>
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On Wed, Jul 06, 2022 at 02:32:25AM +0200, Jason A. Donenfeld wrote:
> When RDRAND was introduced, there was much discussion on whether it
> should be trusted and how the kernel should handle that. Initially, two
> mechanisms cropped up, CONFIG_ARCH_RANDOM, a compile time switch, and
> "nordrand", a boot-time switch.
> 
> Later the thinking evolved. With a properly designed RNG, using RDRAND
> values alone won't harm anything, even if the outputs are malicious.
> Rather, the issue is whether those values are being *trusted* to be good
> or not. And so a new set of options were introduced as the real
> ones that people use -- CONFIG_RANDOM_TRUST_CPU and "random.trust_cpu".
> With these options, RDRAND is used, but it's not always credited. So in
> the worst case, it does nothing, and in the best case, maybe it helps.
> 
> Along the way, CONFIG_ARCH_RANDOM's meaning got sort of pulled into the
> center and became something certain platforms force-select.
> 
> The old options don't really help with much, and it's a bit odd to have
> special handling for these instructions when the kernel can deal fine
> with the existence or untrusted existence or broken existence or
> non-existence of that CPU capability.
> 
> So this commit simplifies things down to the two options that are
> actually used, and removes the confusing old ones that aren't used or
> useful. It leaves "nordrand" for now, as the removal of that will take a
> different route.
> 
> Cc: Catalin Marinas <catalin.marinas@arm.com>
> Cc: Will Deacon <will@kernel.org>
> Cc: Michael Ellerman <mpe@ellerman.id.au>
> Cc: Heiko Carstens <hca@linux.ibm.com>
> Cc: Alexander Gordeev <agordeev@linux.ibm.com>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: H. Peter Anvin <hpa@zytor.com>
> Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> Cc: Arnd Bergmann <arnd@arndb.de>
> Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
...
>  arch/s390/Kconfig                                 | 15 ---------------
>  arch/s390/configs/zfcpdump_defconfig              |  1 -
>  arch/s390/crypto/Makefile                         |  2 +-
>  arch/s390/include/asm/archrandom.h                |  3 ---

For s390:
Acked-by: Heiko Carstens <hca@linux.ibm.com>