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

David Daney
2017-02-27T22:21:21Z

On 02/27/2017 02:09 PM, Steven Rostedt wrote:
> On Mon, 27 Feb 2017 13:41:13 -0800
> David Daney <ddaney@caviumnetworks.com> wrote:
>
>> On 02/27/2017 01:06 PM, Steven Rostedt wrote:
>>> On Mon, 27 Feb 2017 11:59:50 -0800
>>> David Daney <ddaney@caviumnetworks.com> wrote:
>>>
>>>> For me the size is not the important issue, it is the alignment of the
>>>> struct jump_entry entries in the table. I don't understand how your
>>>> patch helps, and I cannot Acked-by unless I understand what is being
>>>> done and can see that it is both correct and necessary.
>>>
>>> You brought up a very good point and I'm glad that I had Jason Cc all
>>> the arch maintainers in one patch.
>>>
>>> I think jump_labels may be much more broken than we think, and Jason's
>>> fix doesn't fix anything. We had this same issues with tracepoints.
>>>
>>> I'm looking at jump_label_init, and how we iterate over an array of
>>> struct jump_entry's that was put together by the linker. The problem is
>>> that jump_entry is not a power of 2 in size.
>>>
>>
>> ELF sections may have an ENTSIZE property exactly for arrays. Since
>> each jump_entry will have a unique value they cannot be merged, but we
>> can tell the assembler they are an array and get them properly packed.
>> Perhaps something like (untested):
>>
>> .pushsection __jump_table, \"awM\", at progbits,24
>> FOO
>> .popsection
>>
>
> And the linker will honor this too?

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.

David Daney
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