linux-kernel - [PATCH -tip 2/3] x86/percpu: Use C for arch_raw_cpu

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Date:   Sun, 15 Oct 2023 22:24:40 +0200
From:   Uros Bizjak <ubizjak@...il.com>
To:     x86@...nel.org, linux-kernel@...r.kernel.org
Cc:     Uros Bizjak <ubizjak@...il.com>, Andy Lutomirski <luto@...nel.org>,
        Brian Gerst <brgerst@...il.com>,
        Denys Vlasenko <dvlasenk@...hat.com>,
        "H . Peter Anvin" <hpa@...or.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Nadav Amit <namit@...are.com>
Subject: [PATCH -tip 2/3] x86/percpu: Use C for arch_raw_cpu_ptr()

Implement arch_raw_cpu_ptr() in C to allow the compiler to perform
better optimizations, such as setting an appropriate base to compute
the address. The compiler is free to choose either MOV or ADD from
this_cpu_off address to construct the optimal final address.

There are some other issues when memory access to the percpu area is
implemented with an asm. Compilers can not eliminate asm common
subexpressions over basic block boundaries, but are extremely good
at optimizing memory access. By implementing arch_raw_cpu_ptr() in C,
the compiler can eliminate additional redundant loads from this_cpu_off,
further reducing the number of percpu offset reads from 1646 to 1631.

Cc: Andy Lutomirski <luto@...nel.org>
Cc: Brian Gerst <brgerst@...il.com>
Cc: Denys Vlasenko <dvlasenk@...hat.com>
Cc: H. Peter Anvin <hpa@...or.com>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: Josh Poimboeuf <jpoimboe@...hat.com>
Co-developed-by: Nadav Amit <namit@...are.com>
Signed-off-by: Nadav Amit <namit@...are.com>
Signed-off-by: Uros Bizjak <ubizjak@...il.com>
---
v2: Use CONFIG_USE_X86_SEG_SUPPORT to handle
    cases where KASAN is enabled.
v3: Split the patch to the part that introduces MOV with
    tcp_ptr__ += (unsigned long)(ptr) and the part that
    introduces named address spaces.
---
 arch/x86/include/asm/percpu.h | 17 +++++++++++++++++
 1 file changed, 17 insertions(+)

diff --git a/arch/x86/include/asm/percpu.h b/arch/x86/include/asm/percpu.h
index 915675f3ad60..54746903b8c3 100644
--- a/arch/x86/include/asm/percpu.h
+++ b/arch/x86/include/asm/percpu.h
@@ -49,6 +49,21 @@
 #define __force_percpu_prefix	"%%"__stringify(__percpu_seg)":"
 #define __my_cpu_offset		this_cpu_read(this_cpu_off)
 
+#ifdef CONFIG_USE_X86_SEG_SUPPORT
+/*
+ * Efficient implementation for cases in which the compiler supports
+ * named address spaces.  Allows the compiler to perform additional
+ * optimizations that can save more instructions.
+ */
+#define arch_raw_cpu_ptr(ptr)					\
+({								\
+	unsigned long tcp_ptr__;				\
+	tcp_ptr__ = __raw_cpu_read(, this_cpu_off);		\
+								\
+	tcp_ptr__ += (unsigned long)(ptr);			\
+	(typeof(*(ptr)) __kernel __force *)tcp_ptr__;		\
+})
+#else /* CONFIG_USE_X86_SEG_SUPPORT */
 /*
  * Compared to the generic __my_cpu_offset version, the following
  * saves one instruction and avoids clobbering a temp register.
@@ -63,6 +78,8 @@
 	tcp_ptr__ += (unsigned long)(ptr);			\
 	(typeof(*(ptr)) __kernel __force *)tcp_ptr__;		\
 })
+#endif /* CONFIG_USE_X86_SEG_SUPPORT */
+
 #else /* CONFIG_SMP */
 #define __percpu_seg_override
 #define __percpu_prefix		""
-- 
2.41.0