linux-kernel - Re: [PATCH v2 01/24] x86/xen: Mark cpu_bringup_and_idle() as dead_end

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Date:   Mon, 30 Aug 2021 09:48:34 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Juergen Gross <jgross@...e.com>
Cc:     Josh Poimboeuf <jpoimboe@...hat.com>, tglx@...utronix.de,
        linux-kernel@...r.kernel.org, joro@...tes.org,
        boris.ostrovsky@...cle.com, x86@...nel.org, mbenes@...e.com,
        rostedt@...dmis.org, dvyukov@...gle.com, elver@...gle.com
Subject: Re: [PATCH v2 01/24] x86/xen: Mark cpu_bringup_and_idle() as
 dead_end_function

On Mon, Aug 30, 2021 at 07:55:02AM +0200, Juergen Gross wrote:
> > > From: Josh Poimboeuf <jpoimboe@...hat.com>
> > > Subject: [PATCH] x86/xen: Move hypercall_page to top of the file
> > > 
> > > Because hypercall_page is page-aligned, the assembler inexplicably adds
> > > an unreachable jump from after the end of the previous code to the
> > > beginning of hypercall_page.
> > > 
> > > That confuses objtool, understandably.  It also creates significant text
> > > fragmentation.  As a result, much of the object file is wasted text
> > > (nops).
> > > 
> > > Move hypercall_page to the beginning of the file to both prevent the
> > > text fragmentation and avoid the dead jump instruction.
> > > 
> > > $ size /tmp/head_64.before.o /tmp/head_64.after.o
> > >     text       data        bss        dec        hex    filename
> > >    10924     307252       4096     322272      4eae0
> > > /tmp/head_64.before.o
> > >     6823     307252       4096     318171      4dadb
> > > /tmp/head_64.after.o
> > > 
> > > Signed-off-by: Josh Poimboeuf <jpoimboe@...hat.com
> > 
> > Reviewed-by: Juergen Gross <jgross@...e.com>
> 
> Umm, will this be carried through the tip tree, or shall I take it in
> the xen tree?

I have it in the whole x86/objtool/paravirt series. I you want it in the
Xen tree, I'll be happy to drop it from there, although I hope to get
all that merged this cycles somewhere.