x86/fpu: Add CONFIG_X86_DEBUG_FPU=y FPU debugging code

There are various internal FPU state debugging checks that never
trigger in practice, but which are useful for FPU code development.

Separate these out into CONFIG_X86_DEBUG_FPU=y, and also add a
couple of new ones.

The size difference is about 0.5K of code on defconfig:

   text        data     bss          filename
   15028906    2578816  1638400      vmlinux
   15029430    2578816  1638400      vmlinux

( Keep this enabled by default until the new FPU code is debugged. )

Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Fenghua Yu <fenghua.yu@intel.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/Kconfig.debug

@@ -332,4 +332,16 @@ config X86_DEBUG_STATIC_CPU_HAS
 
 	  If unsure, say N.
 
+config X86_DEBUG_FPU
+	bool "Debug the x86 FPU code"
+	depends on DEBUG_KERNEL
+	default y
+	---help---
+	  If this option is enabled then there will be extra sanity
+	  checks and (boot time) debug printouts added to the kernel.
+	  This debugging adds some small amount of runtime overhead
+	  to the kernel.
+
+	  If unsure, say N.
+
 endmenu

arch/x86/include/asm/fpu/internal.h

@@ -59,6 +59,15 @@ extern void fpu__clear(struct fpu *fpu);
 extern void fpu__init_check_bugs(void);
 extern void fpu__resume_cpu(void);
 
+/*
+ * Debugging facility:
+ */
+#ifdef CONFIG_X86_DEBUG_FPU
+# define WARN_ON_FPU(x) WARN_ON_ONCE(x)
+#else
+# define WARN_ON_FPU(x) ({ 0; })
+#endif
+
 DECLARE_PER_CPU(struct fpu *, fpu_fpregs_owner_ctx);
 
 /*
@@ -296,6 +305,8 @@ static inline void __fpregs_deactivate_hw(void)
 /* Must be paired with an 'stts' (fpregs_deactivate_hw()) after! */
 static inline void __fpregs_deactivate(struct fpu *fpu)
 {
+	WARN_ON_FPU(!fpu->fpregs_active);
+
 	fpu->fpregs_active = 0;
 	this_cpu_write(fpu_fpregs_owner_ctx, NULL);
 }
@@ -303,6 +314,8 @@ static inline void __fpregs_deactivate(struct fpu *fpu)
 /* Must be paired with a 'clts' (fpregs_activate_hw()) before! */
 static inline void __fpregs_activate(struct fpu *fpu)
 {
+	WARN_ON_FPU(fpu->fpregs_active);
+
 	fpu->fpregs_active = 1;
 	this_cpu_write(fpu_fpregs_owner_ctx, fpu);
 }
@@ -433,9 +446,11 @@ switch_fpu_prepare(struct fpu *old_fpu, struct fpu *new_fpu, int cpu)
 static inline void switch_fpu_finish(struct fpu *new_fpu, fpu_switch_t fpu_switch)
 {
 	if (fpu_switch.preload) {
-		if (unlikely(copy_fpstate_to_fpregs(new_fpu)))
+		if (unlikely(copy_fpstate_to_fpregs(new_fpu))) {
+			WARN_ON_FPU(1);
 			fpu__clear(new_fpu);
 		}
+	}
 }
 
 /*

arch/x86/kernel/fpu/core.c

@@ -38,13 +38,13 @@ DEFINE_PER_CPU(struct fpu *, fpu_fpregs_owner_ctx);
 
 static void kernel_fpu_disable(void)
 {
-	WARN_ON(this_cpu_read(in_kernel_fpu));
+	WARN_ON_FPU(this_cpu_read(in_kernel_fpu));
 	this_cpu_write(in_kernel_fpu, true);
 }
 
 static void kernel_fpu_enable(void)
 {
-	WARN_ON_ONCE(!this_cpu_read(in_kernel_fpu));
+	WARN_ON_FPU(!this_cpu_read(in_kernel_fpu));
 	this_cpu_write(in_kernel_fpu, false);
 }
 
@@ -109,7 +109,7 @@ void __kernel_fpu_begin(void)
 {
 	struct fpu *fpu = &current->thread.fpu;
 
-	WARN_ON_ONCE(!irq_fpu_usable());
+	WARN_ON_FPU(!irq_fpu_usable());
 
 	kernel_fpu_disable();
 
@@ -127,7 +127,7 @@ void __kernel_fpu_end(void)
 	struct fpu *fpu = &current->thread.fpu;
 
 	if (fpu->fpregs_active) {
-		if (WARN_ON(copy_fpstate_to_fpregs(fpu)))
+		if (WARN_ON_FPU(copy_fpstate_to_fpregs(fpu)))
 			fpu__clear(fpu);
 	} else {
 		__fpregs_deactivate_hw();
@@ -187,7 +187,7 @@ EXPORT_SYMBOL_GPL(irq_ts_restore);
  */
 void fpu__save(struct fpu *fpu)
 {
-	WARN_ON(fpu != &current->thread.fpu);
+	WARN_ON_FPU(fpu != &current->thread.fpu);
 
 	preempt_disable();
 	if (fpu->fpregs_active) {
@@ -233,7 +233,7 @@ EXPORT_SYMBOL_GPL(fpstate_init);
  */
 static void fpu_copy(struct fpu *dst_fpu, struct fpu *src_fpu)
 {
-	WARN_ON(src_fpu != &current->thread.fpu);
+	WARN_ON_FPU(src_fpu != &current->thread.fpu);
 
 	/*
 	 * Don't let 'init optimized' areas of the XSAVE area
@@ -284,7 +284,7 @@ int fpu__copy(struct fpu *dst_fpu, struct fpu *src_fpu)
  */
 void fpu__activate_curr(struct fpu *fpu)
 {
-	WARN_ON_ONCE(fpu != &current->thread.fpu);
+	WARN_ON_FPU(fpu != &current->thread.fpu);
 
 	if (!fpu->fpstate_active) {
 		fpstate_init(&fpu->state);
@@ -321,7 +321,7 @@ EXPORT_SYMBOL_GPL(fpu__activate_curr);
  */
 void fpu__activate_stopped(struct fpu *child_fpu)
 {
-	WARN_ON_ONCE(child_fpu == &current->thread.fpu);
+	WARN_ON_FPU(child_fpu == &current->thread.fpu);
 
 	if (child_fpu->fpstate_active) {
 		child_fpu->last_cpu = -1;
@@ -407,7 +407,7 @@ static inline void copy_init_fpstate_to_fpregs(void)
  */
 void fpu__clear(struct fpu *fpu)
 {
-	WARN_ON_ONCE(fpu != &current->thread.fpu); /* Almost certainly an anomaly */
+	WARN_ON_FPU(fpu != &current->thread.fpu); /* Almost certainly an anomaly */
 
 	if (!use_eager_fpu()) {
 		/* FPU state will be reallocated lazily at the first use. */

arch/x86/kernel/fpu/init.c

@@ -143,6 +143,11 @@ EXPORT_SYMBOL_GPL(xstate_size);
  */
 static void __init fpu__init_system_xstate_size_legacy(void)
 {
+	static int on_boot_cpu = 1;
+
+	WARN_ON_FPU(!on_boot_cpu);
+	on_boot_cpu = 0;
+
 	/*
 	 * Note that xstate_size might be overwriten later during
 	 * fpu__init_system_xstate().
@@ -214,7 +219,12 @@ __setup("eagerfpu=", eager_fpu_setup);
  */
 static void __init fpu__init_system_ctx_switch(void)
 {
-	WARN_ON(current->thread.fpu.fpstate_active);
+	static bool on_boot_cpu = 1;
+
+	WARN_ON_FPU(!on_boot_cpu);
+	on_boot_cpu = 0;
+
+	WARN_ON_FPU(current->thread.fpu.fpstate_active);
 	current_thread_info()->status = 0;
 
 	/* Auto enable eagerfpu for xsaveopt */

arch/x86/kernel/fpu/xstate.c

@@ -262,6 +262,11 @@ static void __init setup_xstate_comp(void)
  */
 static void __init setup_init_fpu_buf(void)
 {
+	static int on_boot_cpu = 1;
+
+	WARN_ON_FPU(!on_boot_cpu);
+	on_boot_cpu = 0;
+
 	if (!cpu_has_xsave)
 		return;
 
@@ -317,6 +322,10 @@ static void __init init_xstate_size(void)
 void __init fpu__init_system_xstate(void)
 {
 	unsigned int eax, ebx, ecx, edx;
+	static int on_boot_cpu = 1;
+
+	WARN_ON_FPU(!on_boot_cpu);
+	on_boot_cpu = 0;
 
 	if (!cpu_has_xsave) {
 		pr_info("x86/fpu: Legacy x87 FPU detected.\n");
@@ -324,7 +333,7 @@ void __init fpu__init_system_xstate(void)
 	}
 
 	if (boot_cpu_data.cpuid_level < XSTATE_CPUID) {
-		WARN(1, "x86/fpu: XSTATE_CPUID missing!\n");
+		WARN_ON_FPU(1);
 		return;
 	}