From bd42d3870f6a1bf8cabf93fb3f3d4a25910307bd Mon Sep 17 00:00:00 2001
From: Ian Moffett <ian@osmora.org>
Date: Thu, 20 Nov 2025 18:47:59 -0500
Subject: kern/amd64+sys: Add cacheline alignment macro

Cacheline alignment is a mitigation against false sharing imposed
by locality of a shared resource.

Signed-off-by: Ian Moffett <ian@osmora.org>
---
 sys/arch/amd64/conf/link.ld | 4 ++++
 sys/inc/sys/cdefs.h         | 6 ++++++
 sys/inc/sys/param.h         | 5 +++++
 3 files changed, 15 insertions(+)

diff --git a/sys/arch/amd64/conf/link.ld b/sys/arch/amd64/conf/link.ld
index 9ae1a01..5b42aeb 100644
--- a/sys/arch/amd64/conf/link.ld
+++ b/sys/arch/amd64/conf/link.ld
@@ -33,6 +33,10 @@ SECTIONS
         *(.data)
     } :data
 
+    .cacheline_aligned : ALIGN(64) {
+        *(.cacheline_aligned)
+    }
+
     .bss : {
         *(COMMON)
         *(.bss .bss.*)
diff --git a/sys/inc/sys/cdefs.h b/sys/inc/sys/cdefs.h
index 1da0392..43310f7 100644
--- a/sys/inc/sys/cdefs.h
+++ b/sys/inc/sys/cdefs.h
@@ -30,6 +30,8 @@
 #ifndef _SYS_CDEFS_H_
 #define _SYS_CDEFS_H_ 1
 
+#include <sys/param.h>
+
 #define __asmv          __asm__ __volatile__
 #define __weak          __attribute__((__weak__))
 #define __strong        __attribute__((__strong__))
@@ -48,4 +50,8 @@
 #define __barrier()     __asmv("" ::: "memory")
 #define __static_assert _Static_assert
 
+#define __cacheline_aligned             \
+    __aligned(COHERENCY_UNIT)           \
+    __section(".cacheline_aligned")
+
 #endif  /* !_SYS_CDEFS_H_ */
diff --git a/sys/inc/sys/param.h b/sys/inc/sys/param.h
index 96e270e..f8e939e 100644
--- a/sys/inc/sys/param.h
+++ b/sys/inc/sys/param.h
@@ -32,6 +32,11 @@
 
 #include <sys/types.h>
 
+/* XXX: Should be arch overridable */
+#if !defined(COHERENCY_UNIT)
+#define COHERENCY_UNIT 64
+#endif  /* !COHERENCY_UNIT */
+
 /* Bit related macros */
 #define ISSET(v, f)  ((v) & (f))
 #define BIT(n) (1ULL << (n))
-- 
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