linux-kernel - Re: [PATCH] jump_label: align jump

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Date:   Mon, 27 Feb 2017 14:21:21 -0800
From:   David Daney <ddaney@...iumnetworks.com>
To:     Steven Rostedt <rostedt@...dmis.org>, linux-mips@...ux-mips.org,
        Ralf Baechle <ralf@...ux-mips.org>
Cc:     Jason Baron <jbaron@...mai.com>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linuxppc-dev@...ts.ozlabs.org, Ingo Molnar <mingo@...nel.org>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Anton Blanchard <anton@...ba.org>,
        Rabin Vincent <rabin@....in>,
        Russell King <linux@...linux.org.uk>,
        Chris Metcalf <cmetcalf@...lanox.com>,
        Zhigang Lu <zlu@...hip.com>
Subject: Re: [PATCH] jump_label: align jump_entry table to at least 4-bytes

On 02/27/2017 02:09 PM, Steven Rostedt wrote:
> On Mon, 27 Feb 2017 13:41:13 -0800
> David Daney <ddaney@...iumnetworks.com> wrote:
>
>> On 02/27/2017 01:06 PM, Steven Rostedt wrote:
>>> On Mon, 27 Feb 2017 11:59:50 -0800
>>> David Daney <ddaney@...iumnetworks.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\",@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

View attachment "0001-MIPS-jump_lable-Give-__jump_table-elements-an-entsiz.patch" of type "text/x-patch" (2318 bytes)