linux-kernel - Re: [PATCH v3 1/2] x86: mce: kexec: switch MCE handler for kexec/kdump

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Date:	Tue, 3 Mar 2015 19:53:24 +0100
From:	Borislav Petkov <bp@...en8.de>
To:	Naoya Horiguchi <n-horiguchi@...jp.nec.com>
Cc:	Tony Luck <tony.luck@...el.com>,
	Prarit Bhargava <prarit@...hat.com>,
	Vivek Goyal <vgoyal@...hat.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Junichi Nomura <j-nomura@...jp.nec.com>,
	Kiyoshi Ueda <k-ueda@...jp.nec.com>
Subject: Re: [PATCH v3 1/2] x86: mce: kexec: switch MCE handler for
 kexec/kdump

On Tue, Mar 03, 2015 at 09:01:49AM +0000, Naoya Horiguchi wrote:
> kexec disables (or "shoots down") all CPUs other than a crashing CPU before
> entering the 2nd kernel. But the MCE handler is still enabled after that,
> so if MCE happens and broadcasts over the CPUs after the main thread starts
> the 2nd kernel (which might not initialize MCE device yet, or might decide
> not to enable it,) MCE handler runs only on the other CPUs (not on the main
> thread,) leading to kernel panic with MCE synchronization. The user-visible
> effect of this bug is kdump failure.
> 
> Our standard MCE handler do_machine_check() assumes some about system's
> status and it's hard to alter it to cover kexec/kdump context, so let's add
> another kdump-specific one and switch to it.
> 
> Note that this problem exists since current MCE handler was implemented in
> 2.6.32, and recently commit 716079f66eac ("mce: Panic when a core has reached
> a timeout") made it more visible by changing the default behavior of the
> synchronization timeout from "ignore" to "panic".
> 
> Signed-off-by: Naoya Horiguchi <n-horiguchi@...jp.nec.com>
> Cc: <stable@...r.kernel.org>        [2.6.32+]
> ---
> ChangeLog v2 -> v3
> - go to "switch MCE handler" approach
> 
> ChangeLog v1 -> v2
> - clear MSR_IA32_MCG_CTL, MSR_IA32_MCx_CTL, and CR4.MCE instead of using
>   global flag to ignore MCE events.
> - fixed the description of the problem
> ---
>  arch/x86/include/asm/mce.h       |  6 +++++
>  arch/x86/kernel/cpu/mcheck/mce.c | 47 ++++++++++++++++++++++++++++++++++++++++
>  arch/x86/kernel/crash.c          |  3 +++
>  3 files changed, 56 insertions(+)
> 
> diff --git v3.19.orig/arch/x86/include/asm/mce.h v3.19/arch/x86/include/asm/mce.h
> index 51b26e895933..8010d4b77183 100644
> --- v3.19.orig/arch/x86/include/asm/mce.h
> +++ v3.19/arch/x86/include/asm/mce.h
> @@ -114,6 +114,9 @@ struct mca_config {
>  	int monarch_timeout;
>  	int panic_timeout;
>  	u32 rip_msr;
> +#ifdef CONFIG_KEXEC
> +	int kdump_cpu;
> +#endif

This CONFIG_KEXEC-ifdeffery is too ugly to live. Please put everything
in arch/x86/kernel/crash.c. AFAICT, you don't need to touch anything in
arch/x86/kernel/cpu/mcheck/ for your purposes.

Thanks.

-- 
Regards/Gruss,
    Boris.

ECO tip #101: Trim your mails when you reply.
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