Re: [PATCH] i386 kdump boot cpu physical apicid fix

[Andrew Morton <akpm@osdl.org>]

Vivek Goyal <vgoyal@in.ibm.com> wrote:
>
> Hi,
> 
> Sombody reported following BUG while testing kdump. Please find attached
> the fix.
> 
> ------------[ cut here ]------------
> kernel BUG at arch/i386/kernel/apic.c:447!
> invalid opcode: 0000 [#1]
> Modules linked in:
> CPU:    0
> EIP:    0060:[<c100acc3>]    Not tainted VLI
> EFLAGS: 00010246   (2.6.17-rc4-16M #5)
> EIP is at setup_local_APIC+0x20/0x1a3
> eax: 00000000   ebx: c4e61f88   ecx: 00000000   edx: 00000020
> esi: 00050014   edi: c4e61f88   ebp: 00000000   esp: c4e61f5c
> ds: 007b   es: 007b   ss: 0068
> Process swapper (pid: 1, threadinfo=c4e60000 task=c1389a10)
> Stack: <0>c4e61f88 c133dd44 c13264c0 00000001 00000000 00000000 00000000 00000000
>        00000000 00000000 00000000 00000001 00000000 00000000 00000000 00000000
>        00000000 00000000 00000000 c4e60000 00000000 c1000284 c1002342 c12a6260
> Call Trace:
>  <c13264c0> APIC_init_uniprocessor+0xa9/0xd8   <c1000284> init+0x28/0x1f6
>  <c1002342> ret_from_fork+0x6/0x14   <c100025c> init+0x0/0x1f6
>  <c100025c> init+0x0/0x1f6   <c1000ae5> kernel_thread_helper+0x5/0xb
> Code: 9d 5b 5e c3 a1 18 1c 35 c1 eb e0 56 53 8b 35 30 d0 ff ff a1 20 d0 ff ff c1 e8 18 83 e0 0f 0f a3 05 80 d2 34 c1 19 c0 85 c0 75 08 <0f> 0b bf 01 9b 78 24 c1 c7 05 e0 d0 ff ff ff ff ff ff a1 d0 d0
> EIP: [<c100acc3>] setup_local_APIC+0x20/0x1a3 SS:ESP 0068:c4e61f5c
>  <0>Kernel panic - not syncing: Attempted to kill init!
> 
> =============================================================================
> 
> 
> o Kdump second kernel boot fails after a system crash if second kernel
>   is UP and acpi=off and if crash occurred on a non-boot cpu. 
> 
> o Issue here is that MP tables report boot cpu lapic id as 0 but second
>   kernel is booting on a different processor and MP table data is stale
>   in this context. Hence apic_id_registered() check fails in setup_local_APIC()
>   when called from APIC_init_uniprocessor(). 
> 
> o Problem is not seen if ACPI is enabled as in that case
>   boot_cpu_physical_apicid is read from the LAPIC.
> 
> o Problem is not seen with SMP kernels as well because in this case also
>   boot_cpu_physical_apicid is read from LAPIC. (smp_boot_cpus()).
> 
> o This patch fixes the problem by reading the boot_cpu_physical_apicid
>   from LAPIC for all the cases hence bringing uniformity. At the same time
>   reading from LAPIC should be more reliable then trusting MP tables. My
>   understanding is that MP tables are anyway becoming a thing of past.
> 

Oh dear.  The APIC code is a teetering wreck already.

> 
> diff -puN arch/i386/kernel/apic.c~kdump-i386-boot-cpu-physical-apicid-fix arch/i386/kernel/apic.c
> --- linux-2.6.17-rc4-16M/arch/i386/kernel/apic.c~kdump-i386-boot-cpu-physical-apicid-fix	2006-05-17 13:27:44.000000000 -0400
> +++ linux-2.6.17-rc4-16M-root/arch/i386/kernel/apic.c	2006-05-18 05:11:44.000000000 -0400
> @@ -860,12 +860,7 @@ void __init init_apic_mappings(void)
>  	printk(KERN_DEBUG "mapped APIC to %08lx (%08lx)\n", APIC_BASE,
>  	       apic_phys);
>  
> -	/*
> -	 * Fetch the APIC ID of the BSP in case we have a
> -	 * default configuration (or the MP table is broken).
> -	 */
> -	if (boot_cpu_physical_apicid == -1U)
> -		boot_cpu_physical_apicid = GET_APIC_ID(apic_read(APIC_ID));
> +	boot_cpu_physical_apicid = GET_APIC_ID(apic_read(APIC_ID));
>  

I just don't think we can do this sort of thing.  The workaround for broken
MP tables is one of those nasty things we've gained through many years of
hard-won real-world experience.

If we just go and toss it away like this, all those machines which used to
work will break and there'll be a sad little dribble of regression reports
which everyone cheerily ignores as usual.

So, sorry, nope.  Please find a way to fix kdump while retaining the
broken-MP-table workaround.