From 63117f826b8750fd7f57c16aa7acbb71bab529e4 Mon Sep 17 00:00:00 2001
From: Andreas Kling <kling@serenityos.org>
Date: Sat, 11 Dec 2021 18:59:40 +0100
Subject: [PATCH] Kernel: Simplify 64-bit HPET reads on x86_64

We don't have to worry about racy 32-bit reads when we're reading the
64-bit HPET value using a 64-bit CPU. :^)
---
 Kernel/Time/HPET.cpp | 13 +++++++++++--
 1 file changed, 11 insertions(+), 2 deletions(-)

diff --git a/Kernel/Time/HPET.cpp b/Kernel/Time/HPET.cpp
index 6feb05af86..990661611d 100644
--- a/Kernel/Time/HPET.cpp
+++ b/Kernel/Time/HPET.cpp
@@ -46,8 +46,13 @@ enum class TimerConfiguration : u32 {
 };
 
 struct [[gnu::packed]] HPETRegister {
-    volatile u32 low;
-    volatile u32 high;
+    union {
+        volatile u64 full;
+        struct {
+            volatile u32 low;
+            volatile u32 high;
+        };
+    };
 };
 
 struct [[gnu::packed]] TimerStructure {
@@ -87,6 +92,9 @@ static_assert(AssertSize<HPETRegistersBlock, 0x500>());
 
 static u64 read_register_safe64(const HPETRegister& reg)
 {
+#if ARCH(X86_64)
+    return reg.full;
+#else
     // As per 2.4.7 this reads the 64 bit value in a consistent manner
     // using only 32 bit reads
     u32 low, high = reg.high;
@@ -98,6 +106,7 @@ static u64 read_register_safe64(const HPETRegister& reg)
         high = new_high;
     }
     return ((u64)high << 32) | (u64)low;
+#endif
 }
 
 static HPET* s_hpet;