Re: [PATCH] x86: fix x2apic preenabled system with kexec

[Suresh Siddha <suresh.b.siddha@intel.com>]

On Thu, 2010-07-15 at 00:00 -0700, Yinghai Lu wrote:
> Found one x2apic system kexec loop test failed 
> when CONFIG_NMI_WATCHDOG=y (old) or CONFIG_LOCKUP_DETECTOR=y (current tip)
> 
> first kernel can kexec second kernel, but second kernel can not kexec third one.
> 
> it can be duplicated on another system with BIOS preenabled x2apic.
> First kernel can not kexec second kernel.
> 
> It turns out, when kernel boot with pre-enabled x2apic, it will not execute
> disable_local_APIC on shutdown path.
> 
> when init_apic_mappings() is called in setup_arch, it will skip setting of
> apic_phys when x2apic_mode is set. ( x2apic_mode is much early check_x2apic())
> Then later, disable_local_APIC() will bail out early because !apic_phys.
> 
> So check !x2apic_mode in x2apic_mode in disable_local_APIC with !apic_phys.

Thanks for the nice debug work! As we still have NMI enabled, it looks
like we get a NMI during kexec and as we reset gdt/idt before kexec
launch, we might get a triple fault causing the system to reboot.

> another solution could be updating init_apic_mappings() to set apic_phys even
> for preenabled x2apic system. Actually even for x2apic system, that lapic
> address is mapped already in early stage.

Below patch is the right one. We should probably unmap apic_phys mapping
when x2apic is enabled by the OS.


> BTW: is there any x2apic preenabled system with apicid of boot cpu > 255?

I am not sure. There might be one. Is there any bug which can't handle
this condition?

> 
> Signed-off-by: Yinghai Lu <yinghai@kernel.org>
> Cc: stable@kernel.org

For this patch:

Acked-by: Suresh Siddha <suresh.b.siddha@intel.com>

> ---
>  arch/x86/kernel/apic/apic.c |    2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> Index: linux-2.6/arch/x86/kernel/apic/apic.c
> ===================================================================
> --- linux-2.6.orig/arch/x86/kernel/apic/apic.c
> +++ linux-2.6/arch/x86/kernel/apic/apic.c
> @@ -921,7 +921,7 @@ void disable_local_APIC(void)
>  	unsigned int value;
>  
>  	/* APIC hasn't been mapped yet */
> -	if (!apic_phys)
> +	if (!x2apic_mode && !apic_phys)
>  		return;
>  
>  	clear_local_APIC();