Subject: [v4 PATCH 2/2] [PowerPC] Allow use of SIMD in interrupts from kernel code

From: Shawn Landden <[email-masked]>
Date: Tue, 14 May 2019 22:37:25 -0300

This second patch is separate because it could be wrong,
like I am not sure about how kernel thread migration works,
and it is even allowing simd in preemptible kernel code.

v4: fix build without CONFIG_AVX

Signed-off-by: Shawn Landden <[email-masked]>
---
 arch/powerpc/include/asm/switch_to.h |  15 +---
 arch/powerpc/kernel/process.c        | 117 +++++++++++++++++++--------
 2 files changed, 88 insertions(+), 44 deletions(-)

diff --git a/arch/powerpc/include/asm/switch_to.h b/arch/powerpc/include/asm/switch_to.h
index 5b03d8a82..c79f7d24a 100644
--- a/arch/powerpc/include/asm/switch_to.h
+++ b/arch/powerpc/include/asm/switch_to.h
@@ -30,10 +30,7 @@ extern void enable_kernel_fp(void);
 extern void flush_fp_to_thread(struct task_struct *);
 extern void giveup_fpu(struct task_struct *);
 extern void save_fpu(struct task_struct *);
-static inline void disable_kernel_fp(void)
-{
-	msr_check_and_clear(MSR_FP);
-}
+extern void disable_kernel_fp(void);
 #else
 static inline void save_fpu(struct task_struct *t) { }
 static inline void flush_fp_to_thread(struct task_struct *t) { }
@@ -44,10 +41,7 @@ extern void enable_kernel_altivec(void);
 extern void flush_altivec_to_thread(struct task_struct *);
 extern void giveup_altivec(struct task_struct *);
 extern void save_altivec(struct task_struct *);
-static inline void disable_kernel_altivec(void)
-{
-	msr_check_and_clear(MSR_VEC);
-}
+extern void disable_kernel_altivec(void);
 #else
 static inline void save_altivec(struct task_struct *t) { }
 static inline void __giveup_altivec(struct task_struct *t) { }
@@ -56,10 +50,7 @@ static inline void __giveup_altivec(struct task_struct *t) { }
 #ifdef CONFIG_VSX
 extern void enable_kernel_vsx(void);
 extern void flush_vsx_to_thread(struct task_struct *);
-static inline void disable_kernel_vsx(void)
-{
-	msr_check_and_clear(MSR_FP|MSR_VEC|MSR_VSX);
-}
+extern void disable_kernel_vsx(void);
 #endif
 
 #ifdef CONFIG_SPE
diff --git a/arch/powerpc/kernel/process.c b/arch/powerpc/kernel/process.c
index ef534831f..0136fd132 100644
--- a/arch/powerpc/kernel/process.c
+++ b/arch/powerpc/kernel/process.c
@@ -170,6 +170,29 @@ void __msr_check_and_clear(unsigned long bits)
 EXPORT_SYMBOL(__msr_check_and_clear);
 
 #ifdef CONFIG_PPC_FPU
+/*
+ * Track whether the kernel is using the FPU state
+ * currently.
+ *
+ * This flag is used:
+ *
+ *   - by IRQ context code to potentially use the FPU
+ *     if it's unused.
+ *
+ *   - to debug kernel_fpu/altivec/vsx_begin()/end() correctness
+ */
+static DEFINE_PER_CPU(bool, in_kernel_fpu);
+
+static bool kernel_fpu_disabled(void)
+{
+	return this_cpu_read(in_kernel_fpu);
+}
+
+static bool interrupted_kernel_fpu_idle(void)
+{
+	return !kernel_fpu_disabled();
+}
+
 static void __giveup_fpu(struct task_struct *tsk)
 {
 	unsigned long msr;
@@ -230,7 +253,8 @@ void enable_kernel_fp(void)
 {
 	unsigned long cpumsr;
 
-	WARN_ON(preemptible());
+	WARN_ON_ONCE(this_cpu_read(in_kernel_fpu));
+	this_cpu_write(in_kernel_fpu, true);
 
 	cpumsr = msr_check_and_set(MSR_FP);
 
@@ -251,6 +275,15 @@ void enable_kernel_fp(void)
 }
 EXPORT_SYMBOL(enable_kernel_fp);
 
+void disable_kernel_fp(void)
+{
+	WARN_ON_ONCE(!this_cpu_read(in_kernel_fpu));
+	this_cpu_write(in_kernel_fpu, false);
+
+	msr_check_and_clear(MSR_FP);
+}
+EXPORT_SYMBOL(disable_kernel_fp);
+
 static int restore_fp(struct task_struct *tsk)
 {
 	if (tsk->thread.load_fp || tm_active_with_fp(tsk)) {
@@ -260,6 +293,37 @@ static int restore_fp(struct task_struct *tsk)
 	}
 	return 0;
 }
+
+/*
+ * Were we in user mode when we were
+ * interrupted?
+ *
+ * Doing kernel_altivec/vsx_begin/end() is ok if we are running
+ * in an interrupt context from user mode - we'll just
+ * save the FPU state as required.
+ */
+static bool interrupted_user_mode(void)
+{
+        struct pt_regs *regs = get_irq_regs();
+
+        return regs && user_mode(regs);
+}
+
+/*
+ * Can we use FPU in kernel mode with the
+ * whole "kernel_fpu/altivec/vsx_begin/end()" sequence?
+ *
+ * It's always ok in process context (ie "not interrupt")
+ * but it is sometimes ok even from an irq.
+ */
+bool may_use_simd(void)
+{
+        return !in_interrupt() ||
+                interrupted_user_mode() ||
+                interrupted_kernel_fpu_idle();
+}
+EXPORT_SYMBOL(may_use_simd);
+
 #else
 static int restore_fp(struct task_struct *tsk) { return 0; }
 #endif /* CONFIG_PPC_FPU */
@@ -295,7 +359,8 @@ void enable_kernel_altivec(void)
 {
 	unsigned long cpumsr;
 
-	WARN_ON(preemptible());
+	WARN_ON_ONCE(this_cpu_read(in_kernel_fpu));
+	this_cpu_write(in_kernel_fpu, true);
 
 	cpumsr = msr_check_and_set(MSR_VEC);
 
@@ -316,6 +381,14 @@ void enable_kernel_altivec(void)
 }
 EXPORT_SYMBOL(enable_kernel_altivec);
 
+extern void disable_kernel_altivec(void)
+{
+	WARN_ON_ONCE(!this_cpu_read(in_kernel_fpu));
+	this_cpu_write(in_kernel_fpu, false);
+	msr_check_and_clear(MSR_VEC);
+}
+EXPORT_SYMBOL(disable_kernel_altivec);
+
 /*
  * Make sure the VMX/Altivec register state in the
  * the thread_struct is up to date for task tsk.
@@ -346,35 +419,6 @@ static int restore_altivec(struct task_struct *tsk)
 	return 0;
 }
 
-/*
- * Were we in user mode when we were
- * interrupted?
- *
- * Doing kernel_altivec/vsx_begin/end() is ok if we are running
- * in an interrupt context from user mode - we'll just
- * save the FPU state as required.
- */
-static bool interrupted_user_mode(void)
-{
-	struct pt_regs *regs = get_irq_regs();
-
-	return regs && user_mode(regs);
-}
-
-/*
- * Can we use FPU in kernel mode with the
- * whole "kernel_fpu/altivec/vsx_begin/end()" sequence?
- *
- * It's always ok in process context (ie "not interrupt")
- * but it is sometimes ok even from an irq.
- */
-bool may_use_simd(void)
-{
-	return !in_interrupt() ||
-		interrupted_user_mode();
-}
-EXPORT_SYMBOL(may_use_simd);
-
 #else
 #define loadvec(thr) 0
 static inline int restore_altivec(struct task_struct *tsk) { return 0; }
@@ -411,7 +455,8 @@ void enable_kernel_vsx(void)
 {
 	unsigned long cpumsr;
 
-	WARN_ON(preemptible());
+	WARN_ON_ONCE(this_cpu_read(in_kernel_fpu));
+	this_cpu_write(in_kernel_fpu, true);
 
 	cpumsr = msr_check_and_set(MSR_FP|MSR_VEC|MSR_VSX);
 
@@ -433,6 +478,14 @@ void enable_kernel_vsx(void)
 }
 EXPORT_SYMBOL(enable_kernel_vsx);
 
+void disable_kernel_vsx(void)
+{
+	WARN_ON_ONCE(!this_cpu_read(in_kernel_fpu));
+	this_cpu_write(in_kernel_fpu, false);
+	msr_check_and_clear(MSR_FP|MSR_VEC|MSR_VSX);
+}
+EXPORT_SYMBOL(disable_kernel_vsx);
+
 void flush_vsx_to_thread(struct task_struct *tsk)
 {
 	if (tsk->thread.regs) {
-- 
2.21.0.1020.gf2820cf01a