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

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Message-ID: <20170227170911.2280ca3e@gandalf.local.home>
Date:   Mon, 27 Feb 2017 17:09:11 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     David Daney <ddaney@...iumnetworks.com>
Cc:     Jason Baron <jbaron@...mai.com>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linux-mips@...ux-mips.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>,
        Ralf Baechle <ralf@...ux-mips.org>,
        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 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?

-- Steve