linux-kernel - [PATCH] x86/percpu: Do not use this_cpu_read_stable

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Date: Sun, 24 Mar 2024 22:19:45 +0100
From: Uros Bizjak <ubizjak@...il.com>
To: x86@...nel.org,
	linux-kernel@...r.kernel.org
Cc: Uros Bizjak <ubizjak@...il.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...nel.org>,
	Borislav Petkov <bp@...en8.de>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	"H. Peter Anvin" <hpa@...or.com>
Subject: [PATCH] x86/percpu: Do not use this_cpu_read_stable_8() for 32 bit targets

this_cpu_read_stable() macro uses __pcpu_size_call_return()
that unconditionally calls this_cpu_read_stable_8() also for
32 bit targets. This usage is ivalid as it will result in the
generation of 64 bit MOVQ instruction on 32 bit targets via
percpu_stable_op() macro.

Since there is no generic support for this_cpu_read_stable_8() for
32 bit targets, the patch defines this_cpu_read_stable_8() to
BUILD_BUG() when CONFIG_X86_64 is not defined. This way, we are
sure that this_cpu_read_stable_8() won't actually be used for
32 bit targets, but it is still defined to prevent build failure.

Signed-off-by: Uros Bizjak <ubizjak@...il.com>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: Ingo Molnar <mingo@...nel.org>
Cc: Borislav Petkov <bp@...en8.de>
Cc: Dave Hansen <dave.hansen@...ux.intel.com>
Cc: "H. Peter Anvin" <hpa@...or.com>
---
 arch/x86/include/asm/percpu.h | 13 +++++++++----
 1 file changed, 9 insertions(+), 4 deletions(-)

diff --git a/arch/x86/include/asm/percpu.h b/arch/x86/include/asm/percpu.h
index f6ddbaaf80bc..1f6404e0c428 100644
--- a/arch/x86/include/asm/percpu.h
+++ b/arch/x86/include/asm/percpu.h
@@ -423,10 +423,6 @@ do {									\
  * actually per-thread variables implemented as per-CPU variables and
  * thus stable for the duration of the respective task.
  */
-#define this_cpu_read_stable_1(pcp)	percpu_stable_op(1, "mov", pcp)
-#define this_cpu_read_stable_2(pcp)	percpu_stable_op(2, "mov", pcp)
-#define this_cpu_read_stable_4(pcp)	percpu_stable_op(4, "mov", pcp)
-#define this_cpu_read_stable_8(pcp)	percpu_stable_op(8, "mov", pcp)
 #define this_cpu_read_stable(pcp)	__pcpu_size_call_return(this_cpu_read_stable_, pcp)
 
 #ifdef CONFIG_USE_X86_SEG_SUPPORT
@@ -495,6 +491,10 @@ do {									\
 #define this_cpu_read_const(pcp)	({ BUILD_BUG(); (typeof(pcp))0; })
 #endif /* CONFIG_USE_X86_SEG_SUPPORT */
 
+#define this_cpu_read_stable_1(pcp)	percpu_stable_op(1, "mov", pcp)
+#define this_cpu_read_stable_2(pcp)	percpu_stable_op(2, "mov", pcp)
+#define this_cpu_read_stable_4(pcp)	percpu_stable_op(4, "mov", pcp)
+
 #define raw_cpu_add_1(pcp, val)		percpu_add_op(1, , (pcp), val)
 #define raw_cpu_add_2(pcp, val)		percpu_add_op(2, , (pcp), val)
 #define raw_cpu_add_4(pcp, val)		percpu_add_op(4, , (pcp), val)
@@ -546,6 +546,8 @@ do {									\
  * 32 bit must fall back to generic operations.
  */
 #ifdef CONFIG_X86_64
+#define this_cpu_read_stable_8(pcp)	percpu_stable_op(8, "mov", pcp)
+
 #define raw_cpu_add_8(pcp, val)			percpu_add_op(8, , (pcp), val)
 #define raw_cpu_and_8(pcp, val)			percpu_to_op(8, , "and", (pcp), val)
 #define raw_cpu_or_8(pcp, val)			percpu_to_op(8, , "or", (pcp), val)
@@ -561,6 +563,9 @@ do {									\
 #define this_cpu_xchg_8(pcp, nval)		this_percpu_xchg_op(pcp, nval)
 #define this_cpu_cmpxchg_8(pcp, oval, nval)	percpu_cmpxchg_op(8, volatile, pcp, oval, nval)
 #define this_cpu_try_cmpxchg_8(pcp, ovalp, nval)	percpu_try_cmpxchg_op(8, volatile, pcp, ovalp, nval)
+#else
+/* There is no generic 64 bit read stable operation for 32 bit targets. */
+#define this_cpu_read_stable_8(pcp)    ({ BUILD_BUG(); (typeof(pcp))0; })
 #endif
 
 static __always_inline bool x86_this_cpu_constant_test_bit(unsigned int nr,
-- 
2.42.0