linux-kernel - correct section for cpu_tss

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Date:   Tue, 2 Jan 2018 10:51:38 -0800
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     tj@...nel.org, cl@...ux.com, tglx@...utronix.de, mingo@...hat.com,
        hpa@...or.com, bp@...e.de, luto@...nel.org,
        kirill.shutemov@...ux.intel.com, thgarnie@...gle.com,
        Josh Poimboeuf <jpoimboe@...hat.com>, minipli@...glemail.com,
        me@...ehuey.com, namit@...are.com, tklauser@...tanz.ch,
        thomas.lendacky@....com
Cc:     LKML <linux-kernel@...r.kernel.org>, x86@...nel.org
Subject: correct section for cpu_tss_rw?

(emailing the folks listed from running `./scripts/get_maintainer.pl
-f` on arch/x86/kernel/process.c, arch/x86/include/asm/processor.h,
and include/linux/percpu-defs.h)

Clang emits the following warning:

arch/x86/kernel/process.c:50:11: warning: section does not match
previous declaration [-Wsection]
__visible DEFINE_PER_CPU_SHARED_ALIGNED(struct tss_struct, cpu_tss_rw) = {
          ^
./include/linux/percpu-defs.h:144:2: note: expanded from macro
'DEFINE_PER_CPU_SHARED_ALIGNED'
        DEFINE_PER_CPU_SECTION(type, name, PER_CPU_SHARED_ALIGNED_SECTION) \
        ^
./include/linux/percpu-defs.h:104:2: note: expanded from macro
'DEFINE_PER_CPU_SECTION'
        __PCPU_ATTRS(sec) PER_CPU_DEF_ATTRIBUTES                        \
        ^
./include/linux/percpu-defs.h:49:26: note: expanded from macro '__PCPU_ATTRS'
        __percpu __attribute__((section(PER_CPU_BASE_SECTION sec)))     \
                                ^
./arch/x86/include/asm/processor.h:365:1: note: previous attribute is here
DECLARE_PER_CPU_PAGE_ALIGNED(struct tss_struct, cpu_tss_rw);
^
./include/linux/percpu-defs.h:159:2: note: expanded from macro
'DECLARE_PER_CPU_PAGE_ALIGNED'
        DECLARE_PER_CPU_SECTION(type, name, "..page_aligned")           \
        ^
./include/linux/percpu-defs.h:101:9: note: expanded from macro
'DECLARE_PER_CPU_SECTION'
        extern __PCPU_ATTRS(sec) __typeof__(type) name
               ^
./include/linux/percpu-defs.h:49:26: note: expanded from macro '__PCPU_ATTRS'
        __percpu __attribute__((section(PER_CPU_BASE_SECTION sec)))     \
                                ^

it seems that from commit c482feefe1a ("x86/entry/64: Make
cpu_entry_area.tss read-only") that cpu_tss_rw is declared but then
defined in two different sections. (Though, it looks like this issue
predates that commit).

In include/linux/percpu-defs.h, there's two pairs of macros:

DECLARE_PER_CPU_SHARED_ALIGNED
DEFINE_PER_CPU_SHARED_ALIGNED
DECLARE_PER_CPU_PAGE_ALIGNED
DEFINE_PER_CPU_PAGE_ALIGNED

It seems that cpu_tss_rw is defined as SHARED_ALIGNED, but then
declared as PAGE_ALIGNED.  Should be an easy fix (that I'm happy to
author), but what section *should* cpu_tss_rw be in (SHARED_ALIGNED or
PAGE_ALIGNED)?  That affects whether I fix the declaration or
definition (and thus the .h or the .c file).

>From the comment in arch/x86/kernel/process.c#50:
 43 /*
 44  * per-CPU TSS segments. Threads are completely 'soft' on Linux,
 45  * no more per-task TSS's. The TSS size is kept cacheline-aligned
 46  * so they are allowed to end up in the .data..cacheline_aligned
 47  * section. Since TSS's are completely CPU-local, we want them
 48  * on exact cacheline boundaries, to eliminate cacheline
ping-pong.
 49  */
 50 __visible DEFINE_PER_CPU_SHARED_ALIGNED(struct tss_struct, cpu_tss_rw) = {

I suspect that cache-line alignment is stricter than page alignment,
so the declaration should be fixed, but I was not sure and wanted to
check?

-- 
Thanks,
~Nick Desaulniers