Re: [PATCH v2 1/2] x86: mce: kexec: turn off MCE in kexec

[Prarit Bhargava <prarit@redhat.com>]

On 02/26/2015 11:58 PM, 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 around CPUs after the main thread starts the
> 2nd kernel (which might not start MCE yet, or might decide not to start MCE,)
> 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.
> 
> 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".
> 
> This patch adds a global variable representing that the system is running
> kdump code in order to "turn off" the MCE handling code in kdump context.
> 
> Signed-off-by: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
> Cc: <stable@vger.kernel.org>        [2.6.32+]
> ---
> 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       |  1 +
>  arch/x86/kernel/cpu/mcheck/mce.c | 17 +++++++++++++++++
>  arch/x86/kernel/crash.c          |  8 ++++++++
>  3 files changed, 26 insertions(+)
> 
> diff --git v3.19.orig/arch/x86/include/asm/mce.h v3.19/arch/x86/include/asm/mce.h
> index 51b26e895933..7ae9927d781a 100644
> --- v3.19.orig/arch/x86/include/asm/mce.h
> +++ v3.19/arch/x86/include/asm/mce.h
> @@ -175,6 +175,7 @@ static inline void mce_amd_feature_init(struct cpuinfo_x86 *c) { }
>  #endif
>  
>  int mce_available(struct cpuinfo_x86 *c);
> +void cpu_emergency_mce_disable(void);
>  
>  DECLARE_PER_CPU(unsigned, mce_exception_count);
>  DECLARE_PER_CPU(unsigned, mce_poll_count);
> diff --git v3.19.orig/arch/x86/kernel/cpu/mcheck/mce.c v3.19/arch/x86/kernel/cpu/mcheck/mce.c
> index 3112b79ace8e..10359ae1f558 100644
> --- v3.19.orig/arch/x86/kernel/cpu/mcheck/mce.c
> +++ v3.19/arch/x86/kernel/cpu/mcheck/mce.c
> @@ -2105,6 +2105,23 @@ static void mce_syscore_shutdown(void)
>  }
>  
>  /*
> + * Called in kdump entering code to turn off MCE handling function. We clear
> + * global switch first to forbid the situation where only portion of CPUs are
> + * responsive to MCE and MCE causes kernel panic with synchronization timeout.
> + */
> +void cpu_emergency_mce_disable(void)
> +{
> +	u64 cap;
> +	int i;
> +
> +	rdmsrl(MSR_IA32_MCG_CAP, cap);
> +	if (cap & MCG_CTL_P)
> +		wrmsr(MSR_IA32_MCG_CTL, 0, 0);
> +	mce_disable_error_reporting();
> +	clear_in_cr4(X86_CR4_MCE);
> +}
> +
> +/*
>   * On resume clear all MCE state. Don't want to see leftovers from the BIOS.
>   * Only one CPU is active at this time, the others get re-added later using
>   * CPU hotplug:
> diff --git v3.19.orig/arch/x86/kernel/crash.c v3.19/arch/x86/kernel/crash.c
> index aceb2f90c716..22451c687fca 100644
> --- v3.19.orig/arch/x86/kernel/crash.c
> +++ v3.19/arch/x86/kernel/crash.c
> @@ -34,6 +34,7 @@
>  #include <asm/cpu.h>
>  #include <asm/reboot.h>
>  #include <asm/virtext.h>
> +#include <asm/mce.h>
>  
>  /* Alignment required for elf header segment */
>  #define ELF_CORE_HEADER_ALIGN   4096
> @@ -112,6 +113,8 @@ static void kdump_nmi_callback(int cpu, struct pt_regs *regs)
>  #endif
>  	crash_save_cpu(regs, cpu);
>  
> +	cpu_emergency_mce_disable();
> +
>  	/*
>  	 * VMCLEAR VMCSs loaded on all cpus if needed.
>  	 */
> @@ -157,6 +160,11 @@ void native_machine_crash_shutdown(struct pt_regs *regs)
>  	/* The kernel is broken so disable interrupts */
>  	local_irq_disable();
>  
> +	/*
> +	 * We can't expect MCE handling to work any more, so turn it off.
> +	 */
> +	cpu_emergency_mce_disable();

What if the system is actually having problems with MCE errors -- which are
leading to system panics of some sort.  Do you *really* want the system to
continue on at that point?

P.

> +
>  	kdump_nmi_shootdown_cpus();
>  
>  	/*
>