linux-kernel - Re: [lkp] [ACPI] 7494b07eba: Kernel panic - not syncing: Watchdog detected hard LOCKUP on cpu 0

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Message-ID: <56169B98.3070105@linaro.org>
Date:	Thu, 8 Oct 2015 10:36:40 -0600
From:	Al Stone <al.stone@...aro.org>
To:	Hanjun Guo <hanjun.guo@...aro.org>,
	kernel test robot <ying.huang@...ux.intel.com>
Cc:	lkp@...org, LKML <linux-kernel@...r.kernel.org>,
	"Rafael J. Wysocki" <rafael.j.wysocki@...el.com>
Subject: Re: [lkp] [ACPI] 7494b07eba: Kernel panic - not syncing: Watchdog
 detected hard LOCKUP on cpu 0

On 10/08/2015 05:44 AM, Hanjun Guo wrote:
> On 10/08/2015 11:21 AM, kernel test robot wrote:
>> FYI, we noticed the below changes on
>>
>> https://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git master
>> commit 7494b07ebaae2117629024369365f7be7adc16c3 ("ACPI: add in a
>> bad_madt_entry() function to eventually replace the macro")
>>
>> [    0.000000] ACPI: undefined MADT subtable type for FADT 4.0: 127 (length 12)
> 
> Seems that the MADT table contains reserved subtable type (0x7F),
> so this is traded as a wrong type in our patch.
> 
>> [    0.000000] ACPI: Error parsing LAPIC address override entry
> 
> This was called by early_acpi_parse_madt_lapic_addr_ovr() in
> arch/x86/kernel/acpi/boot.c, which is scanning MADT for the first
> time when booting, so it will fail the boot process when finding
> the reserved MADT subtable type.
> 
>> [    0.000000] ACPI: Invalid BIOS MADT, disabling ACPI
> 
> As the spec said in Table 5-46 (ACPI 6.0):
> 
> 0x10-0x7F Reserved. OSPM skips structures of the reserved type.
> 
> Should we just ignore those reserved type when scanning the MADT
> table? In the patch "ACPI: add in a bad_madt_entry() function to
> eventually replace the macro", we just trade it as wrong, that's
> why we failed to boot the system.
> 
> Thanks
> Hanjun

Arrgh.  This is why people get frustrated with ACPI.  The spec is
saying that those sub-table types are reserved -- implying they can
and probably will be used for something else in the future -- but
then vendors are shipping firmware that uses those reserved values,
and an OS *expects* them to be used, and there is *no* documentation
of it other than a kernel workaround.

So yet again, technically this MADT subtable *is* wrong, and someone
should slap the vendor for doing this.  But, the practical side of
this is that we now have to workaround what is now a known violation
of the spec.

The more ACPI allows this kind of nonsense, the less usable it will
become.

At a minimum, whoever is responsible for this firmware needs to make
sure the spec reflects what they are doing.  In the meantime, the
only option is what Hanjun suggests -- make this a warning and not a
failure.  I'll prepare a patch and attach it to a reply here in a few
minutes...

-- 
ciao,
al
-----------------------------------
Al Stone
Software Engineer
Linaro Enterprise Group
al.stone@...aro.org
-----------------------------------
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