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

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Date:   Thu, 27 Sep 2018 22:26:56 -0400
From:   Masayoshi Mizuma <msys.mizuma@...il.com>
To:     Borislav Petkov <bp@...en8.de>
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 11:17:47PM +0200, Borislav Petkov wrote:
> 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.
> 
> :-)

You are right, I didn't make it clear enough...
Thank you for your comments, I'll fix the description.

Thanks!
Masa