kernel: Move current_fp field out of z_kernel

The current_fp field in the z_kernel structure is only used
by 32-bit x86 (which does not support SMP). As such, it should
reside in the arch specific of section of _kernel.cpus[0].

This also changes the name of 'current_fp' to 'fpu_owner' to
be more consistent with other architectures.

Signed-off-by: Peter Mitsis <peter.mitsis@intel.com>

arch/x86/core/ia32/float.c

@@ -194,7 +194,7 @@ void z_float_enable(struct k_thread *thread, unsigned int options)
 	 * must be preserved).
 	 */
 
-	fp_owner = _kernel.current_fp;
+	fp_owner = _kernel.cpus[0].arch.fpu_owner;
 	if (fp_owner != NULL) {
 		if ((fp_owner->arch.flags & X86_THREAD_FLAG_ALL) != 0) {
 			FpCtxSave(fp_owner);
@@ -215,7 +215,7 @@ void z_float_enable(struct k_thread *thread, unsigned int options)
 		 * (The FP context is "live" in hardware, not saved in TCS.)
 		 */
 
-		_kernel.current_fp = thread;
+		_kernel.cpus[0].arch.fpu_owner = thread;
 	} else {
 		/*
 		 * When enabling FP support for someone else, assign ownership
@@ -230,7 +230,7 @@ void z_float_enable(struct k_thread *thread, unsigned int options)
 			 * to its original state.
 			 */
 
-			_kernel.current_fp = thread;
+			_kernel.cpus[0].arch.fpu_owner = thread;
 			z_FpAccessDisable();
 		} else {
 			/*
@@ -280,10 +280,10 @@ int z_float_disable(struct k_thread *thread)
 
 	if (thread == _current) {
 		z_FpAccessDisable();
-		_kernel.current_fp = (struct k_thread *)0;
+		_kernel.cpus[0].arch.fpu_owner = (struct k_thread *)0;
 	} else {
-		if (_kernel.current_fp == thread) {
-			_kernel.current_fp = (struct k_thread *)0;
+		if (_kernel.cpus[0].arch.fpu_owner == thread) {
+			_kernel.cpus[0].arch.fpu_owner = (struct k_thread *)0;
 		}
 	}
 

arch/x86/core/ia32/swap.S

@@ -50,7 +50,7 @@
  * Floating point registers are handled using a lazy save/restore mechanism
  * since it's expected relatively few threads will be created with the
  * K_FP_REGS or K_SSE_REGS option bits.  The kernel data structure maintains a
- * 'current_fp' field to keep track of the thread that "owns" the floating
+ * 'fpu_owner' field to keep track of the thread that "owns" the floating
  * point registers.  Floating point registers consist of ST0->ST7 (x87 FPU and
  * MMX registers) and XMM0 -> XMM7.
  *
@@ -176,7 +176,7 @@ SECTION_FUNC(PINNED_TEXT, arch_swap)
 	 * If so, there there is no need to restore the floating point context.
 	 */
 
-	movl	_kernel_offset_to_current_fp(%edi), %ebx
+	movl	_kernel_offset_to_fpu_owner(%edi), %ebx
 	cmpl	%ebx, %eax
 	je	restoreContext_NoFloatSwap
 
@@ -265,7 +265,7 @@ restoreContext_NoFloatRestore:
 
 	/* record that the incoming thread "owns" the floating point registers */
 
-	movl	%eax, _kernel_offset_to_current_fp(%edi)
+	movl	%eax, _kernel_offset_to_fpu_owner(%edi)
 
 
 	/*

arch/x86/core/offsets/ia32_offsets.c

@@ -33,6 +33,12 @@
 GEN_OFFSET_SYM(_thread_arch_t, excNestCount);
 #endif
 
+#if defined(CONFIG_FPU_SHARING)
+GEN_OFFSET_SYM(_kernel_t, cpus);
+GEN_OFFSET_SYM(_cpu_t, arch);
+GEN_OFFSET_SYM(_cpu_arch_t, fpu_owner);
+#endif
+
 #ifdef CONFIG_USERSPACE
 GEN_OFFSET_SYM(_thread_arch_t, psp);
 #ifndef CONFIG_X86_COMMON_PAGE_TABLE

arch/x86/include/ia32/offsets_short_arch.h

@@ -14,6 +14,9 @@
 #define _kernel_offset_to_isf \
 	(___kernel_t_arch_OFFSET + ___kernel_arch_t_isf_OFFSET)
 
+#define _kernel_offset_to_fpu_owner \
+	(___kernel_t_cpus_OFFSET + ___cpu_t_arch_OFFSET + ___cpu_arch_t_fpu_owner_OFFSET)
+
 /* end - kernel */
 
 /* threads */

include/zephyr/arch/structs.h

@@ -29,6 +29,8 @@
 #include <zephyr/arch/riscv/structs.h>
 #elif defined(CONFIG_ARM)
 #include <zephyr/arch/arm/structs.h>
+#elif defined(CONFIG_X86) && !defined(CONFIG_X86_64)
+#include <zephyr/arch/x86/ia32/structs.h>
 #else
 
 /* Default definitions when no architecture specific definitions exist. */

include/zephyr/arch/x86/ia32/structs.h

@@ -0,0 +1,33 @@
+/*
+ * Copyright (c) 2025 Intel
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+#ifndef ZEPHYR_INCLUDE_X86_STRUCTS_H_
+#define ZEPHYR_INCLUDE_X86_STRUCTS_H_
+
+#include <stdint.h>
+
+struct k_thread;
+
+/* Per CPU architecture specifics (empty) */
+struct _cpu_arch {
+
+#if defined(CONFIG_FPU_SHARING)
+	/*
+	 * A 'sse_owner' field does not exist in addition to the 'fpu_owner'
+	 * field since it's not possible to divide the IA-32 non-integer
+	 * registers into 2 distinct blocks owned by differing threads.  In
+	 * other words, given that the 'fxnsave/fxrstor' instructions
+	 * save/restore both the X87 FPU and XMM registers, it's not possible
+	 * for a thread to only "own" the XMM registers.
+	 */
+
+	struct k_thread *fpu_owner;
+#elif defined(__cplusplus)
+	/* Ensure this struct does not have a size of 0 which is not allowed in C++. */
+	uint8_t dummy;
+#endif
+};
+
+#endif /* ZEPHYR_INCLUDE_X86_STRUCTS_H_ */

include/zephyr/kernel_structs.h

@@ -217,20 +217,6 @@ struct z_kernel {
 	struct _ready_q ready_q;
 #endif
 
-#ifdef CONFIG_FPU_SHARING
-	/*
-	 * A 'current_sse' field does not exist in addition to the 'current_fp'
-	 * field since it's not possible to divide the IA-32 non-integer
-	 * registers into 2 distinct blocks owned by differing threads.  In
-	 * other words, given that the 'fxnsave/fxrstor' instructions
-	 * save/restore both the X87 FPU and XMM registers, it's not possible
-	 * for a thread to only "own" the XMM registers.
-	 */
-
-	/* thread that owns the FP regs */
-	struct k_thread *current_fp;
-#endif
-
 #if defined(CONFIG_THREAD_MONITOR)
 	struct k_thread *threads; /* singly linked list of ALL threads */
 #endif

kernel/include/kernel_offsets.h

@@ -48,10 +48,6 @@ GEN_OFFSET_SYM(_kernel_t, ready_q);
 GEN_OFFSET_SYM(_ready_q_t, cache);
 #endif /* CONFIG_SMP */
 
-#ifdef CONFIG_FPU_SHARING
-GEN_OFFSET_SYM(_kernel_t, current_fp);
-#endif /* CONFIG_FPU_SHARING */
-
 GEN_OFFSET_SYM(_thread_base_t, user_options);
 
 GEN_OFFSET_SYM(_thread_t, base);

kernel/include/offsets_short.h

@@ -34,9 +34,6 @@
 #define _kernel_offset_to_idle \
 	(___kernel_t_idle_OFFSET)
 
-#define _kernel_offset_to_current_fp \
-	(___kernel_t_current_fp_OFFSET)
-
 #define _kernel_offset_to_ready_q_cache \
 	(___kernel_t_ready_q_OFFSET + ___ready_q_t_cache_OFFSET)