linux-kernel - Re: [PATCH 1/2] x86: Make E820_X_MAX unconditionally larger than E820MAX

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Date:   Wed, 30 Nov 2016 07:21:48 +0100
From:   Ingo Molnar <mingo@...nel.org>
To:     Alex Thorlton <athorlton@....com>
Cc:     linux-kernel@...r.kernel.org, Russ Anderson <rja@....com>,
        David Vrabel <david.vrabel@...rix.com>,
        Ingo Molnar <mingo@...hat.com>,
        Juergen Gross <jgross@...e.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        Denys Vlasenko <dvlasenk@...hat.com>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>, x86@...nel.org,
        xen-devel@...ts.xenproject.org
Subject: Re: [PATCH 1/2] x86: Make E820_X_MAX unconditionally larger than
 E820MAX


* Alex Thorlton <athorlton@....com> wrote:

> It's really not necessary to limit E820_X_MAX to 128 in the non-EFI
> case.  This commit drops E820_X_MAX's dependency on CONFIG_EFI, so that
> E820_X_MAX is always at least slightly larger than E820MAX.
> 
> The real motivation behind this is actually to prevent some issues in
> the Xen kernel, where the XENMEM_machine_memory_map hypercall can
> produce an e820 map larger than 128 entries, even on systems where the
> original e820 table was quite a bit smaller than that, depending on how
> many IOAPICs are installed on the system.
> 
> Signed-off-by: Alex Thorlton <athorlton@....com>
> Suggested-by: Ingo Molnar <mingo@...hat.com>
> Cc: Russ Anderson <rja@....com>
> Cc: David Vrabel <david.vrabel@...rix.com>
> Cc: Ingo Molnar <mingo@...hat.com>
> Cc: Juergen Gross <jgross@...e.com>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: "H. Peter Anvin" <hpa@...or.com>
> Cc: Denys Vlasenko <dvlasenk@...hat.com>
> Cc: Boris Ostrovsky <boris.ostrovsky@...cle.com>
> Cc: x86@...nel.org
> Cc: xen-devel@...ts.xenproject.org
> ---
>  arch/x86/include/asm/e820.h | 10 ++++++----
>  1 file changed, 6 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/x86/include/asm/e820.h b/arch/x86/include/asm/e820.h
> index 476b574..aa00d33 100644
> --- a/arch/x86/include/asm/e820.h
> +++ b/arch/x86/include/asm/e820.h
> @@ -1,13 +1,15 @@
>  #ifndef _ASM_X86_E820_H
>  #define _ASM_X86_E820_H
>  
> -#ifdef CONFIG_EFI
> +/*
> + * We need to make sure that E820_X_MAX is defined
> + * before we include uapi/asm/e820.h
> + */
>  #include <linux/numa.h>
>  #define E820_X_MAX (E820MAX + 3 * MAX_NUMNODES)

What we need an explanation for in the comment is what does this stand for (what 
does the 'X' mean?), and what is the magic 3*MAX_NUMNODES about?

Thanks,

	Ingo