linux-kernel - Re: [PATCH v2 1/2] x86/mm: Identify the end of the kernel area to be reserved

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Message-ID: <20190617104740.GG27127@zn.tnic>
Date:   Mon, 17 Jun 2019 12:47:40 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     "Lendacky, Thomas" <Thomas.Lendacky@....com>
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "x86@...nel.org" <x86@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Baoquan He <bhe@...hat.com>, Lianbo Jiang <lijiang@...hat.com>
Subject: Re: [PATCH v2 1/2] x86/mm: Identify the end of the kernel area to be
 reserved

On Fri, Jun 14, 2019 at 09:15:18PM +0000, Lendacky, Thomas wrote:
> The memory occupied by the kernel is reserved using memblock_reserve()
> in setup_arch(). Currently, the area is from symbols _text to __bss_stop.
> Everything after __bss_stop must be specifically reserved otherwise it
> is discarded. This is not clearly documented.

Hmm, so I see this in arch/x86/kernel/vmlinux.lds.S after _end:

        _end = .;

        STABS_DEBUG
        DWARF_DEBUG

        /* Sections to be discarded */
        DISCARDS
        /DISCARD/ : {
                *(.eh_frame)
        }

and over DISCARDS:

/*
 * Default discarded sections.
 *
 * Some archs want to discard exit text/data at runtime rather than
 * link time due to cross-section references such as alt instructions,
 * bug table, eh_frame, etc.  DISCARDS must be the last of output
 * section definitions so that such archs put those in earlier section
 * definitions.
 */
#define DISCARDS

That sounds like it is documented to me, or do you mean something else?

> Add a new symbol, __end_of_kernel_reserve, that more readily identifies
> what is reserved, along with comments that indicate what is reserved,
> what is discarded and what needs to be done to prevent a section from
> being discarded.
> 
> Cc: Baoquan He <bhe@...hat.com>
> Cc: Lianbo Jiang <lijiang@...hat.com>
> Signed-off-by: Tom Lendacky <thomas.lendacky@....com>
> ---
>  arch/x86/include/asm/sections.h | 2 ++
>  arch/x86/kernel/setup.c         | 8 +++++++-
>  arch/x86/kernel/vmlinux.lds.S   | 9 ++++++++-
>  3 files changed, 17 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/include/asm/sections.h b/arch/x86/include/asm/sections.h
> index 8ea1cfdbeabc..71b32f2570ab 100644
> --- a/arch/x86/include/asm/sections.h
> +++ b/arch/x86/include/asm/sections.h
> @@ -13,4 +13,6 @@ extern char __end_rodata_aligned[];
>  extern char __end_rodata_hpage_align[];
>  #endif
>  
> +extern char __end_of_kernel_reserve[];
> +
>  #endif	/* _ASM_X86_SECTIONS_H */
> diff --git a/arch/x86/kernel/setup.c b/arch/x86/kernel/setup.c
> index 08a5f4a131f5..32eb70625b3b 100644
> --- a/arch/x86/kernel/setup.c
> +++ b/arch/x86/kernel/setup.c
> @@ -827,8 +827,14 @@ dump_kernel_offset(struct notifier_block *self, unsigned long v, void *p)
>  
>  void __init setup_arch(char **cmdline_p)
>  {
> +	/*
> +	 * Reserve the memory occupied by the kernel between _text and
> +	 * __end_of_kernel_reserve symbols. Any kernel sections after the
> +	 * __end_of_kernel_reserve symbol must be explicity reserved with a
> +	 * separate memblock_reserve() or it will be discarded.

s/it/they/

> +	 */
>  	memblock_reserve(__pa_symbol(_text),
> -			 (unsigned long)__bss_stop - (unsigned long)_text);
> +			 (unsigned long)__end_of_kernel_reserve - (unsigned long)_text);
>  
>  	/*
>  	 * Make sure page 0 is always reserved because on systems with
> diff --git a/arch/x86/kernel/vmlinux.lds.S b/arch/x86/kernel/vmlinux.lds.S
> index 0850b5149345..ca2252ca6ad7 100644
> --- a/arch/x86/kernel/vmlinux.lds.S
> +++ b/arch/x86/kernel/vmlinux.lds.S
> @@ -368,6 +368,14 @@ SECTIONS
>  		__bss_stop = .;
>  	}
>  
> +	/*
> +	 * The memory occupied from _text to here, __end_of_kernel_reserve, is
> +	 * automatically reserved in setup_arch(). Anything after here must be
> +	 * explicitly reserved using memblock_reserve() or it will be discarded
> +	 * and treated as available memory.
> +	 */
> +	__end_of_kernel_reserve = .;
> +
>  	. = ALIGN(PAGE_SIZE);
>  	.brk : AT(ADDR(.brk) - LOAD_OFFSET) {
>  		__brk_base = .;
> @@ -382,7 +390,6 @@ SECTIONS
>  	STABS_DEBUG
>  	DWARF_DEBUG
>  
> -	/* Sections to be discarded */

Huh?

They're called DISCARD* ...

>       DISCARDS
>       /DISCARD/ : {
>               *(.eh_frame)

-- 
Regards/Gruss,
    Boris.

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