linux-kernel - [PATCH 1/3] x86/common: Align cpu_caps_cleared and cpu_caps

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Message-Id: <20190916223958.27048-2-tony.luck@intel.com>
Date:   Mon, 16 Sep 2019 15:39:56 -0700
From:   Tony Luck <tony.luck@...el.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Fenghua Yu <fenghua.yu@...el.com>, Borislav Petkov <bp@...e.de>,
        Tony Luck <tony.luck@...el.com>,
        "Ingo Molnar" <mingo@...hat.com>, "Borislav Petkov" <bp@...en8.de>,
        "H Peter Anvin" <hpa@...or.com>,
        "Peter Zijlstra" <peterz@...radead.org>,
        "Andrew Morton" <akpm@...ux-foundation.org>,
        "Dave Hansen" <dave.hansen@...el.com>,
        "Paolo Bonzini" <pbonzini@...hat.com>,
        "Radim Krcmar" <rkrcmar@...hat.com>,
        "Sai Praneeth Prakhya" <sai.praneeth.prakhya@...el.com>,
        "Ravi V Shankar" <ravi.v.shankar@...el.com>,
        "linux-kernel" <linux-kernel@...r.kernel.org>,
        "x86" <x86@...nel.org>
Subject: [PATCH 1/3] x86/common: Align cpu_caps_cleared and cpu_caps_set to unsigned long

From: Fenghua Yu <fenghua.yu@...el.com>

cpu_caps_cleared[] and cpu_caps_set[] may not be aligned to unsigned long.
Atomic operations (i.e. set_bit() and clear_bit()) on the bitmaps may
access two cache lines (a.k.a. split lock) and cause the CPU to do a bus
lock to block all memory accesses from other processors to ensure
atomicity.

To avoid the overall performance degradation from the bus locking, align
the two variables to unsigned long.

Defining the variables as unsigned long may also fix the issue because
they will be naturally aligned to unsigned long. But that needs additional
code changes. Adding __aligned(unsigned long) is a simpler fix.

Signed-off-by: Fenghua Yu <fenghua.yu@...el.com>
Reviewed-by: Borislav Petkov <bp@...e.de>
Signed-off-by: Tony Luck <tony.luck@...el.com>
---
 arch/x86/kernel/cpu/common.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
index f125bf7ecb6f..87627091f45f 100644
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -565,8 +565,9 @@ static const char *table_lookup_model(struct cpuinfo_x86 *c)
 	return NULL;		/* Not found */
 }
 
-__u32 cpu_caps_cleared[NCAPINTS + NBUGINTS];
-__u32 cpu_caps_set[NCAPINTS + NBUGINTS];
+/* Aligned to unsigned long to avoid split lock in atomic bitmap ops */
+__u32 cpu_caps_cleared[NCAPINTS + NBUGINTS] __aligned(sizeof(unsigned long));
+__u32 cpu_caps_set[NCAPINTS + NBUGINTS] __aligned(sizeof(unsigned long));
 
 void load_percpu_segment(int cpu)
 {
-- 
2.20.1