linux-kernel - Re: [PATCH 2/3] ACPI: add BAD_MADT_GICC

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Date:	Fri, 12 Jun 2015 11:20:42 -0600
From:	Al Stone <al.stone@...aro.org>
To:	Lorenzo Pieralisi <lorenzo.pieralisi@....com>
CC:	"linux-acpi@...r.kernel.org" <linux-acpi@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linaro-acpi@...ts.linaro.org" <linaro-acpi@...ts.linaro.org>,
	"patches@...aro.org" <patches@...aro.org>,
	"linaro-kernel@...ts.linaro.org" <linaro-kernel@...ts.linaro.org>
Subject: Re: [PATCH 2/3] ACPI: add BAD_MADT_GICC_ENTRY() macro

On 06/12/2015 08:52 AM, Lorenzo Pieralisi wrote:
> On Thu, Jun 11, 2015 at 08:45:10PM +0100, al.stone@...aro.org wrote:
>> From: Al Stone <al.stone@...aro.org>
>>
>> The BAD_MADT_ENTRY() macro is designed to work for all of the subtables
>> of the MADT.  In the ACPI 5.1 version of the spec, the struct for the
>> GICC subtable (struct acpi_madt_generic_interrupt) is 76 bytes long; in
>> ACPI 6.0, the struct is 80 bytes long.  But, there is only one definition
>> in ACPICA for this struct -- and that is the 6.0 version.  Hence, when
>> BAD_MADT_ENTRY() compares the struct size to the length in the GICC
>> subtable, it fails if 5.1 structs are in use, and there are systems in
>> the wild that have them.
>>
>> This patch adds the BAD_MADT_GICC_ENTRY() that checks the GICC subtable
>> only, accounting for the difference in specification versions that are
>> possible.  The BAD_MADT_ENTRY() will continue to work as is for all other
>> MADT subtables.
> 
> Unfortunately that's nothing new, it seems. ia64 put in place a quite
> nifty solution to that (I *guess* owing to ACPI 3.0 updates to Local
> sapic specs), have a look at:
> 
> arch/ia64/kernel/acpi.c acpi_parse_lsacpi()
> 
> /*Skip BAD_MADT_ENTRY check, as lsapic size could vary */
> 
> We remove the check, job done ;-)

Heh.  Yes, that's one way to do it :).

>> Signed-off-by: Al Stone <al.stone@...aro.org>
>> Reviewed-by: Hanjun Guo <hanjun.guo@...aro.org>
>> Reviewed-by: Graeme Gregory <graeme.gregory@...aro.org>
>>
>> CC: Rafael J. Wysocki <rjw@...ysocki.net>
>> CC: Len Brown <lenb@...nel.org>
>>
>> ---
>>  include/linux/acpi.h | 7 +++++++
>>  1 file changed, 7 insertions(+)
>>
>> diff --git a/include/linux/acpi.h b/include/linux/acpi.h
>> index 33ed313..8a83f91 100644
>> --- a/include/linux/acpi.h
>> +++ b/include/linux/acpi.h
>> @@ -127,6 +127,13 @@ static inline void acpi_initrd_override(void *data, size_t size)
>>  		(!entry) || (unsigned long)entry + sizeof(*entry) > end ||  \
>>  		((struct acpi_subtable_header *)entry)->length < sizeof(*entry))
>>  
>> +#define BAD_MADT_GICC_ENTRY(entry, end) (				    \
>> +		(!entry) || (unsigned long)entry + sizeof(*entry) > end ||  \
>> +		((ACPI_FADT_SPEC_VERSION == ACPI_SPEC_VERSION(5,1)) &&	    \
>> +		 (entry->header.length != 76))				||  \
>> +		((ACPI_FADT_SPEC_VERSION == ACPI_SPEC_VERSION(6,0)) &&	    \
>> +		 (entry->header.length != 80)))
> 
> I would make those length magic numbers ACPICA defines at least.

Yup, you're right.  I'll fix that.

> It is not a GICC only issue, that's true for all MADT subtables that change
> size with versions so, maybe we can replace the sizeof(*entry) in
> BAD_MADT_ENTRY with a macro compound statement returning the subtable
> length (where you can add a switch case on entry->type and return
> sizeof(*entry) in the default case) ?
> 
> Overkill ? Certainly ugly, but at least you do not need to patch anything
> else.

There is precedent for this; there's a function to print MADT subtable
entries with exactly this structure.  And yeah, it's a bit ugly, but it
does the job.

> I am inclined to relegate these checks to ACPICA tools (statically)
> altogether.

Interestingly enough, the acpi_table_print_madt_entry() function I was
referring to is in the drivers/acpi code, not ACPICA.  I wonder what the
history is on that and why it ended up there...

> It is better to check Len and Rafael opinion on this first before coding it.
> 
> Lorenzo
> 

Yup.  Agreed.

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