linux-kernel - Re: [PATCH V2 03/20] x86: Add entry trampolines to kcore

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Date:   Thu, 17 May 2018 10:52:30 -0700
From:   Dave Hansen <dave.hansen@...ux.intel.com>
To:     Adrian Hunter <adrian.hunter@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Arnaldo Carvalho de Melo <acme@...nel.org>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Andy Lutomirski <luto@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>, Andi Kleen <ak@...ux.intel.com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Joerg Roedel <joro@...tes.org>, Jiri Olsa <jolsa@...hat.com>,
        linux-kernel@...r.kernel.org, x86@...nel.org
Subject: Re: [PATCH V2 03/20] x86: Add entry trampolines to kcore

On 05/17/2018 02:21 AM, Adrian Hunter wrote:
> From: Alexander Shishkin <alexander.shishkin@...ux.intel.com>
> 
> Signed-off-by: Alexander Shishkin <alexander.shishkin@...ux.intel.com>

For the benefit of the poor souls looking at this code in the decades to
come, could you please take a few minutes to write a couple of sentences
about why this is being done?

> diff --git a/arch/x86/mm/cpu_entry_area.c b/arch/x86/mm/cpu_entry_area.c
> index d1da5cf4b2de..fb1fbc8538fa 100644
> --- a/arch/x86/mm/cpu_entry_area.c
> +++ b/arch/x86/mm/cpu_entry_area.c
> @@ -3,6 +3,7 @@
>  #include <linux/spinlock.h>
>  #include <linux/percpu.h>
>  #include <linux/kallsyms.h>
> +#include <linux/kcore.h>
>  
>  #include <asm/cpu_entry_area.h>
>  #include <asm/pgtable.h>
> @@ -14,6 +15,7 @@
>  #ifdef CONFIG_X86_64
>  static DEFINE_PER_CPU_PAGE_ALIGNED(char, exception_stacks
>  	[(N_EXCEPTION_STACKS - 1) * EXCEPTION_STKSZ + DEBUG_STKSZ]);
> +static DEFINE_PER_CPU(struct kcore_list, kcore_entry_trampoline);
>  #endif
>  
>  struct cpu_entry_area *get_cpu_entry_area(int cpu)
> @@ -147,6 +149,9 @@ static void __init setup_cpu_entry_area(int cpu)
>  
>  	cea_set_pte(&get_cpu_entry_area(cpu)->entry_trampoline,
>  		     __pa_symbol(_entry_trampoline), PAGE_KERNEL_RX);
> +	kclist_add(&per_cpu(kcore_entry_trampoline, cpu),
> +		   &get_cpu_entry_area(cpu)->entry_trampoline, PAGE_SIZE,
> +		   KCORE_TEXT);
>  #endif

Any reason not to add a comment like:

/*
 * The cpu_entry_area alias addresses are not in the kernel binary
 * so they do not show up in /proc/kcore normally.  This adds entries
 * for them manually.
 */