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* [PATCH] x86/percpu: Unify arch_raw_cpu_ptr() defines
@ 2024-03-22 10:27 Uros Bizjak
  2024-03-22 10:59 ` [tip: x86/percpu] " tip-bot2 for Uros Bizjak
  0 siblings, 1 reply; 2+ messages in thread
From: Uros Bizjak @ 2024-03-22 10:27 UTC (permalink / raw)
  To: x86, linux-kernel
  Cc: Uros Bizjak, Ingo Molnar, Andy Lutomirski, Brian Gerst,
	Denys Vlasenko, H . Peter Anvin, Linus Torvalds, Peter Zijlstra,
	Thomas Gleixner, Josh Poimboeuf

When building a 32 bit VDSO for a 64 bit kernel, games are played with
CONFIG_X86_64. {this,raw}_cpu_read_8() macros are conditionally defined
on CONFIG_X86_64 and when CONFIG_X86_64 is undefined in fake_32bit_build.h
various build failures in generic percpu header files can happen. To make
things worse, the build of 32 bit VDSO for a 64 bit kernel grew dependency
on arch_raw_cpu_ptr() macro and the build fails if arch_raw_cpu_ptr()
macro is not defined.

To mitigate these issues, x86 carefully defines arch_raw_cpu_ptr() to
avoid any dependency on raw_cpu_read_8() and thus CONFIG_X86_64. W/o
segment register support, the definition uses size-agnostic MOV asm
mnemonic and hopes that _ptr argument won't ever be 64 bit size on
32 bit targets (although newer GCCs warn for this situation with
"unsupported size for integer register"), and w/ segment register
support the definition uses size-agnostic __raw_cpu_read() macro.

Fortunately, raw_cpu_read() is not used in 32 bit VDSO for a 64 bit kernel.
However, we can't simply omit the definition of arch_raw_cpu_read(),
since the build will fail when building vdso/vdso32/vclock_gettime.o.

The patch defines arch_raw_cpu_ptr to BUILD_BUG() when BUILD_VDSO32_64
macro is defined. This way, we are sure that arch_raw_cpu_ptr() won't
actually be used in 32 bit VDSO for a 64 bit kernel, but it is still
defined to prevent build failure.

Finally, we can unify arch_raw_cpu_ptr() between builds w/ and w/o
x86 segment register support, substituting two tricky macro definitions
with a straightforward implementation.

There is no size difference and no difference in number of this_cpu_off
accesses between patched and unpatched kernel when the kernel is built
either w/ and w/o segment register support.

Signed-off-by: Uros Bizjak <ubizjak@gmail.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
---
 arch/x86/include/asm/percpu.h | 42 +++++++++++++++--------------------
 1 file changed, 18 insertions(+), 24 deletions(-)

diff --git a/arch/x86/include/asm/percpu.h b/arch/x86/include/asm/percpu.h
index 7563e69838c4..39d394ce1fca 100644
--- a/arch/x86/include/asm/percpu.h
+++ b/arch/x86/include/asm/percpu.h
@@ -59,36 +59,30 @@
 #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 */
+#ifdef CONFIG_X86_64
+#define __raw_my_cpu_offset	raw_cpu_read_8(this_cpu_off);
+#else
+#define __raw_my_cpu_offset	raw_cpu_read_4(this_cpu_off);
+#endif
+
 /*
  * Compared to the generic __my_cpu_offset version, the following
  * saves one instruction and avoids clobbering a temp register.
+ *
+ * arch_raw_cpu_ptr should not be used in 32 bit VDSO for a 64 bit
+ * kernel, because games are played with CONFIG_X86_64 there and
+ * sizeof(this_cpu_off) becames 4.
  */
-#define arch_raw_cpu_ptr(ptr)					\
+#ifndef BUILD_VDSO32_64
+#define arch_raw_cpu_ptr(_ptr)					\
 ({								\
-	unsigned long tcp_ptr__;				\
-	asm ("mov " __percpu_arg(1) ", %0"			\
-	     : "=r" (tcp_ptr__)					\
-	     : "m" (__my_cpu_var(this_cpu_off)));		\
-								\
-	tcp_ptr__ += (unsigned long)(ptr);			\
-	(typeof(*(ptr)) __kernel __force *)tcp_ptr__;		\
+	unsigned long tcp_ptr__ = __raw_my_cpu_offset;		\
+	tcp_ptr__ += (unsigned long)(_ptr);			\
+	(typeof(*(_ptr)) __kernel __force *)tcp_ptr__;		\
 })
-#endif /* CONFIG_USE_X86_SEG_SUPPORT */
+#else
+#define arch_raw_cpu_ptr(_ptr) ({ BUILD_BUG(); (typeof(_ptr))0; })
+#endif
 
 #define PER_CPU_VAR(var)	%__percpu_seg:(var)__percpu_rel
 
-- 
2.44.0


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