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Date:   Fri, 22 Jul 2022 14:06:58 +0200
From:   Heiko Carstens <hca@linux.ibm.com>
To:     "Jason A. Donenfeld" <Jason@zx2c4.com>
Cc:     linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org,
        linuxppc-dev@lists.ozlabs.org, linux-s390@vger.kernel.org,
        x86@kernel.org, Will Deacon <will@kernel.org>,
        Alexander Gordeev <agordeev@linux.ibm.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        "H . Peter Anvin" <hpa@zytor.com>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Borislav Petkov <bp@suse.de>,
        Johannes Berg <johannes@sipsolutions.net>,
        Mark Rutland <mark.rutland@arm.com>,
        Harald Freudenberger <freude@linux.ibm.com>,
        Michael Ellerman <mpe@ellerman.id.au>,
        Holger Dengler <dengler@linux.ibm.com>
Subject: Re: [PATCH v3] random: handle archrandom with multiple longs
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References: <CAHmME9qTA90=GEr6h1GZh0CjS+6tpe5uuqkYoJVv79h0zd0w1w@mail.gmail.com>
 <20220719130207.147536-1-Jason@zx2c4.com>
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On Tue, Jul 19, 2022 at 03:02:07PM +0200, Jason A. Donenfeld wrote:
> The archrandom interface was originally designed for x86, which supplies
> RDRAND/RDSEED for receiving random words into registers, resulting in
> one function to generate an int and another to generate a long. However,
> other architectures don't follow this.
> 
> On arm64, the SMCCC TRNG interface can return between 1 and 3 longs. On
> s390, the CPACF TRNG interface can return arbitrary amounts, with 32
> longs having the same cost as one. On UML, the os_getrandom() interface
> can return arbitrary amounts.
> 
> So change the api signature to take a "max_longs" parameter designating
> the maximum number of longs requested, and then return the number of
> longs generated.
> 
> Since callers need to check this return value and loop anyway, each arch
> implementation does not bother implementing its own loop to try again to
> fill the maximum number of longs. Additionally, all existing callers
> pass in a constant max_longs parameter. Taken together, these two things
> mean that the codegen doesn't really change much for one-word-at-a-time
> platforms, while performance is greatly improved on platforms such as
> s390.
> 
> Cc: Will Deacon <will@kernel.org>
> Cc: Alexander Gordeev <agordeev@linux.ibm.com>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: H. Peter Anvin <hpa@zytor.com>
> Cc: Catalin Marinas <catalin.marinas@arm.com>
> Cc: Borislav Petkov <bp@suse.de>
> Cc: Heiko Carstens <hca@linux.ibm.com>
> Cc: Johannes Berg <johannes@sipsolutions.net>
> Cc: Mark Rutland <mark.rutland@arm.com>
> Cc: Harald Freudenberger <freude@linux.ibm.com>
> Acked-by: Michael Ellerman <mpe@ellerman.id.au>
> Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
> ---
>  arch/arm64/include/asm/archrandom.h   | 102 ++++++++++++--------------
>  arch/arm64/kernel/kaslr.c             |   2 +-
>  arch/powerpc/include/asm/archrandom.h |  30 ++------
>  arch/powerpc/kvm/book3s_hv.c          |   2 +-
>  arch/s390/include/asm/archrandom.h    |  29 ++------
>  arch/um/include/asm/archrandom.h      |  21 ++----
>  arch/x86/include/asm/archrandom.h     |  41 +----------
>  arch/x86/kernel/espfix_64.c           |   2 +-
>  drivers/char/random.c                 |  45 ++++++++----
>  include/asm-generic/archrandom.h      |  18 +----
>  include/linux/random.h                |  12 +--
>  11 files changed, 116 insertions(+), 188 deletions(-)

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