linux-kernel - [PATCH v4 1/5] x86/cacheinfo: move shared cache map definitions

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Date:   Sat,  2 Jul 2022 18:08:24 +0200
From:   Sander Vanheule <sander@...nheule.net>
To:     x86@...nel.org, linux-kernel@...r.kernel.org
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        elver@...gle.com, gregkh@...uxfoundation.org,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>, vschneid@...hat.com,
        Yury Norov <yury.norov@...il.com>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "H . Peter Anvin" <hpa@...or.com>,
        Sander Vanheule <sander@...nheule.net>
Subject: [PATCH v4 1/5] x86/cacheinfo: move shared cache map definitions

The maps to keep track of shared caches between CPUs on SMP systems are
declared in asm/smp.h, among them specifically cpu_llc_shared_map.
These maps are externally defined in cpu/smpboot.c. The latter is only
compiled on CONFIG_SMP=y, which means the declared extern symbols from
asm/smp.h do not have a corresponding definition on uniprocessor builds.

The inline cpu_llc_shared_mask() function from asm/smp.h refers to the
map declaration mentioned above. This function is referenced in
cacheinfo.c inside for_each_cpu() loop macros, to provide cpumask for
the loop. On uniprocessor builds, the symbol for the cpu_llc_shared_map
does not exist. However, the current implementation of for_each_cpu()
also (wrongly) ignores the provided mask.

By sheer luck, the compiler thus optimises out this unused reference to
cpu_llc_shared_map, and the linker therefore does not require the
cpu_llc_shared_mask to actually exist on uniprocessor builds. Only on
SMP bulids does smpboot.o exist to provide the required symbols.

To no longer rely on compiler optimisations for successful uniprocessor
builds, move the definitions of cpu_llc_shared_map and
cpu_l2c_shared_map from smpboot.c to cacheinfo.c.

Signed-off-by: Sander Vanheule <sander@...nheule.net>
---
 arch/x86/kernel/cpu/cacheinfo.c | 6 ++++++
 arch/x86/kernel/smpboot.c       | 4 ----
 2 files changed, 6 insertions(+), 4 deletions(-)

diff --git a/arch/x86/kernel/cpu/cacheinfo.c b/arch/x86/kernel/cpu/cacheinfo.c
index fe98a1465be6..66556833d7af 100644
--- a/arch/x86/kernel/cpu/cacheinfo.c
+++ b/arch/x86/kernel/cpu/cacheinfo.c
@@ -29,6 +29,12 @@
 #define LVL_3		4
 #define LVL_TRACE	5

+/* Shared last level cache maps */
+DEFINE_PER_CPU_READ_MOSTLY(cpumask_var_t, cpu_llc_shared_map);
+
+/* Shared L2 cache maps */
+DEFINE_PER_CPU_READ_MOSTLY(cpumask_var_t, cpu_l2c_shared_map);
+
 struct _cache_table {
 	unsigned char descriptor;
 	char cache_type;
diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c
index 5e7f9532a10d..f24227bc3220 100644
--- a/arch/x86/kernel/smpboot.c
+++ b/arch/x86/kernel/smpboot.c
@@ -95,10 +95,6 @@ EXPORT_PER_CPU_SYMBOL(cpu_core_map);
 DEFINE_PER_CPU_READ_MOSTLY(cpumask_var_t, cpu_die_map);
 EXPORT_PER_CPU_SYMBOL(cpu_die_map);

-DEFINE_PER_CPU_READ_MOSTLY(cpumask_var_t, cpu_llc_shared_map);
-
-DEFINE_PER_CPU_READ_MOSTLY(cpumask_var_t, cpu_l2c_shared_map);
-
 /* Per CPU bogomips and other parameters */
 DEFINE_PER_CPU_READ_MOSTLY(struct cpuinfo_x86, cpu_info);
 EXPORT_PER_CPU_SYMBOL(cpu_info);
-- 
2.36.1