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Date:   Sun, 21 Apr 2019 20:24:36 +0200
From:   Ingo Molnar <mingo@kernel.org>
To:     Dave Young <dyoung@redhat.com>
Cc:     Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        "H. Peter Anvin" <hpa@zytor.com>, x86@kernel.org,
        linux-kernel@vger.kernel.org, vgoyal@redhat.com, bhe@redhat.com,
        piliu@redhat.com, Yinghai Lu <yinghai@kernel.org>,
        Eric Biederman <ebiederm@xmission.com>
Subject: Re: [PATCH 1/2] X86/kdump: move crashkernel=X to reserve under 4G by
 default
Message-ID: <20190421182436.GB35603@gmail.com>
References: <20190421035058.943630505@redhat.com>
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* Dave Young <dyoung@redhat.com> wrote:

> The kdump crashkernel low reservation is limited to under 896M even for
> X86_64. This obscure and miserable limitation exists for old kexec-tools
> compatibility, but the reason is not documented anywhere.
> 
> Some more tests/investigations about the background:
> a) Previously old kexec-tools can only load purgatory to memory under 2G,
>    Eric remove that limitation in 2012 in kexec-tools:
>    Commit b4f9f8599679 ("kexec x86_64: Make purgatory relocatable anywhere
>    in the 64bit address space.")
> 
> b) back in 2013 Yinghai removed all the limitations in new kexec-tools,
>    bzImage64 can be loaded to anywhere.
>    Commit 82c3dd2280d2 ("kexec, x86_64: Load bzImage64 above 4G")
> 
> c) test results with old kexec-tools with old and latest kernels.
>   1. old kexec-tools can not build with modern toolchain anymore,
>      I built it in a RHEL6 vm
>   2. 2.0.0 kexec-tools does not work with latest kernel even with
>      memory under 896M and give an error:
>      "ELF core (kcore) parse failed", it needs below kexec-tools fix 
>      Commit ed15ba1b9977 ("build_mem_phdrs(): check if p_paddr is invalid")
>   3. even with patched kexec-tools which fixes 2),  it still needs some
>      other fixes to work correctly for kaslr enabled kernels.
> 
> So the situation is:
> * old kexec-tools is already broken with latest kernels
> * we can not keep this limitations forever just for compatibility of very
>   old kexec-tools.
> * If one must use old tools then he/she can choose crashkernel=X@Y
> * people have reported bugs crashkernel=384M failed because kaslr makes
>   the 0-896M space sparse, 
> * crashkernel can reserve in low or high area, it is natural to understand 
>   low as memory under 4G
> 
> Hence drop the 896M limitation, and change crashkernel low reservation to
> reserve under 4G by default.
> 
> Signed-off-by: Dave Young <dyoung@redhat.com>
> ---
>  arch/x86/kernel/setup.c |   10 +++++-----
>  1 file changed, 5 insertions(+), 5 deletions(-)
> 
> --- linux-x86.orig/arch/x86/kernel/setup.c
> +++ linux-x86/arch/x86/kernel/setup.c
> @@ -71,6 +71,7 @@
>  #include <linux/tboot.h>
>  #include <linux/jiffies.h>
>  #include <linux/mem_encrypt.h>
> +#include <linux/sizes.h>
>  
>  #include <linux/usb/xhci-dbgp.h>
>  #include <video/edid.h>
> @@ -448,18 +449,17 @@ static void __init memblock_x86_reserve_
>  #ifdef CONFIG_KEXEC_CORE
>  
>  /* 16M alignment for crash kernel regions */
> -#define CRASH_ALIGN		(16 << 20)
> +#define CRASH_ALIGN		SZ_16M
>  
>  /*
>   * Keep the crash kernel below this limit.  On 32 bits earlier kernels
>   * would limit the kernel to the low 512 MiB due to mapping restrictions.
> - * On 64bit, old kexec-tools need to under 896MiB.
>   */
>  #ifdef CONFIG_X86_32
> -# define CRASH_ADDR_LOW_MAX	(512 << 20)
> -# define CRASH_ADDR_HIGH_MAX	(512 << 20)
> +# define CRASH_ADDR_LOW_MAX	SZ_512M
> +# define CRASH_ADDR_HIGH_MAX	SZ_512M
>  #else
> -# define CRASH_ADDR_LOW_MAX	(896UL << 20)
> +# define CRASH_ADDR_LOW_MAX	SZ_4G
>  # define CRASH_ADDR_HIGH_MAX	MAXMEM
>  #endif

Reviewed-by: Ingo Molnar <mingo@kernel.org>

Thanks,

	Ingo