Re: [PATCH] jump_label: align jump_entry table to at least 4-bytes

From: David Daney <ddaney@caviumnetworks.com>
To: Jason Baron <jbaron@akamai.com>,
	Steven Rostedt <rostedt@goodmis.org>,
	Sachin Sant <sachinp@linux.vnet.ibm.com>
Cc: linux-mips@linux-mips.org, Ralf Baechle <ralf@linux-mips.org>,
	linux-arm-kernel@lists.infradead.org,
	linux-kernel@vger.kernel.org, linuxppc-dev@lists.ozlabs.org,
	Ingo Molnar <mingo@kernel.org>,
	Benjamin Herrenschmidt <benh@kernel.crashing.org>,
	Paul Mackerras <paulus@samba.org>,
	Michael Ellerman <mpe@ellerman.id.au>,
	Anton Blanchard <anton@samba.org>, Rabin Vincent <rabin@rab.in>,
	Russell King <linux@armlinux.org.uk>,
	Chris Metcalf <cmetcalf@mellanox.com>,
	Zhigang Lu <zlu@ezchip.com>
Subject: Re: [PATCH] jump_label: align jump_entry table to at least 4-bytes
Date: Mon, 27 Feb 2017 15:34:44 -0800	[thread overview]
Message-ID: <8fb467e8-2ab9-e52b-ea47-083947b9cfe4@caviumnetworks.com> (raw)
In-Reply-To: <68fe24ea-7795-24d8-211b-9d8a50affe9f@akamai.com>

On 02/27/2017 02:50 PM, Jason Baron wrote:
>
>
> On 02/27/2017 05:45 PM, David Daney wrote:
>> On 02/27/2017 02:36 PM, Steven Rostedt wrote:
>>> On Mon, 27 Feb 2017 14:21:21 -0800
>>> David Daney <ddaney@caviumnetworks.com> wrote:
>>>
>>>> See attached for mips.  It seems to do the right thing.
>>>>
>>>> I leave it as an exercise to the reader to fix the other architectures.
>>>>
>>>> Consult your own  binutils experts to verify that what I say is true.
>>>
>>> It may still just be safer to do the pointers instead. That way we
>>> don't need to worry about some strange arch or off by one binutils
>>> messing it up.
>>
>> Obviously it is your choice, but this is bog standard ELF linking.  In
>> theory even the arrays of power-of-2 sized objects should also supply an
>> entity size.  Think __ex_table and its ilk.
>>
>>
>> The benefit of supplying an entsize is that you don't have to change the
>> structure of the existing code and risk breaking something in the
>> process.
>>
>> David Daney
>>
>>
>
> Thanks for the suggestion! I would like to see if this resolves the ppc
> issue we had. I'm attaching a powerpc patch based on your suggestion.
> Hopefully, Sachin can try it.
>

If there are problems, you could try something like:

$ find . -name \*\.o | xargs mips64-octeon-linux-gnu-readelf -eW  | grep 
'File:\| __jump_table'
File: ./drivers/firmware/built-in.o
File: ./drivers/built-in.o
   [3249] __jump_table      PROGBITS        0000000000000000 1838c8 
0022c8 18 WAM  0   0  8
File: ./drivers/spi/built-in.o
   [82] __jump_table      PROGBITS        0000000000000000 008cb0 000048 
18 WAM  0   0  8
File: ./drivers/spi/spi-cavium-octeon.o
File: ./drivers/spi/spi-cavium.o
File: ./drivers/spi/spi.o
.
.
.

Look for files where the size of the __jump_table section is not a 
integer multiple of the entsize.

> Thanks,
>
> -Jason