linux-kernel - Re: [PATCH v4 3/3] docs: kernel-parameters.txt: document rand_mem_physical

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Date:   Thu, 27 Sep 2018 23:17:47 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Masayoshi Mizuma <msys.mizuma@...il.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
        Baoquan He <bhe@...hat.com>,
        Masayoshi Mizuma <m.mizuma@...fujitsu.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 3/3] docs: kernel-parameters.txt: document
 rand_mem_physical_padding parameter

On Thu, Sep 27, 2018 at 04:31:46PM -0400, Masayoshi Mizuma wrote:
> From: Masayoshi Mizuma <m.mizuma@...fujitsu.com>
> 
> This kernel parameter allows to change the padding used
> for the physical memory mapping section when KASLR
> memory is enabled.
> 
> For memory hotplug capable systems, the default padding size,
> CONFIG_RANDOMIZE_MEMORY_PHYSICAL_PADDING, may not be enough.
> The option is useful to adjust the padding size.
> 
> Signed-off-by: Masayoshi Mizuma <m.mizuma@...fujitsu.com>
> ---
>  Documentation/admin-guide/kernel-parameters.txt | 7 +++++++
>  1 file changed, 7 insertions(+)
> 
> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> index 92eb1f4..de43cdf 100644
> --- a/Documentation/admin-guide/kernel-parameters.txt
> +++ b/Documentation/admin-guide/kernel-parameters.txt
> @@ -3529,6 +3529,13 @@
>  			fully seed the kernel's CRNG. Default is controlled
>  			by CONFIG_RANDOM_TRUST_CPU.
>  
> +	rand_mem_physical_padding=
> +			[KNL] Define the padding size in terabytes
> +			used for the physical memory mapping section
> +			when KASLR memory is enabled.
> +			The default value is
> +			CONFIG_RANDOMIZE_MEMORY_PHYSICAL_PADDING.

Yet another kernel parameter which forces me to go look at what the
code does because this help text doesn't really help. And I see that in
previous iterations ok lkml it was *actually* properly explained why
this parameter is needed.

So please summarize that explanation here so that the user can make an
informed decision when reading this help text. Always think of explaning
this to a colleague of yours who doesn't know about the memory padding
and memory hotadd problematic and try to write it in such a way so that
your colleague understands it.

:-)

Thx.

-- 
Regards/Gruss,
    Boris.

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