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

[Shawn Landden <shawn@git.icu>]

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.

Signed-off-by: Shawn Landden <shawn@git.icu>
---
 arch/powerpc/include/asm/simd.h      |  8 +++++
 arch/powerpc/include/asm/switch_to.h | 10 ++----
 arch/powerpc/kernel/process.c        | 50 ++++++++++++++++++++++++++--
 3 files changed, 57 insertions(+), 11 deletions(-)

diff --git a/arch/powerpc/include/asm/simd.h b/arch/powerpc/include/asm/simd.h
index 2c02ad531..7b582b07e 100644
--- a/arch/powerpc/include/asm/simd.h
+++ b/arch/powerpc/include/asm/simd.h
@@ -7,7 +7,15 @@
  * It's always ok in process context (ie "not interrupt")
  * but it is sometimes ok even from an irq.
  */
+#ifdef CONFIG_ALTIVEC
+extern bool irq_simd_usable(void);
 static __must_check inline bool may_use_simd(void)
 {
 	return irq_simd_usable();
 }
+#else
+static inline bool may_use_simd(void)
+{
+	return false;
+}
+#endif
diff --git a/arch/powerpc/include/asm/switch_to.h b/arch/powerpc/include/asm/switch_to.h
index 5b03d8a82..537998997 100644
--- a/arch/powerpc/include/asm/switch_to.h
+++ b/arch/powerpc/include/asm/switch_to.h
@@ -44,10 +44,7 @@ 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 +53,7 @@ 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 e436d708a..41a0ab500 100644
--- a/arch/powerpc/kernel/process.c
+++ b/arch/powerpc/kernel/process.c
@@ -267,6 +267,29 @@ static int restore_fp(struct task_struct *tsk) { return 0; }
 #ifdef CONFIG_ALTIVEC
 #define loadvec(thr) ((thr).load_vec)
 
+/*
+ * Track whether the kernel is using the SIMD state
+ * currently.
+ *
+ * This flag is used:
+ *
+ *   - by IRQ context code to potentially use the FPU
+ *     if it's unused.
+ *
+ *   - to debug kernel_altivec/vsx_begin()/end() correctness
+ */
+static DEFINE_PER_CPU(bool, in_kernel_simd);
+
+static bool kernel_simd_disabled(void)
+{
+	return this_cpu_read(in_kernel_simd);
+}
+
+static bool interrupted_kernel_simd_idle(void)
+{
+	return !kernel_simd_disabled();
+}
+
 static void __giveup_altivec(struct task_struct *tsk)
 {
 	unsigned long msr;
@@ -295,7 +318,9 @@ void enable_kernel_altivec(void)
 {
 	unsigned long cpumsr;
 
-	WARN_ON(preemptible());
+	WARN_ON_ONCE(preemptible());
+	WARN_ON_ONCE(this_cpu_read(in_kernel_simd));
+	this_cpu_write(in_kernel_simd, true);
 
 	cpumsr = msr_check_and_set(MSR_VEC);
 
@@ -316,6 +341,14 @@ void enable_kernel_altivec(void)
 }
 EXPORT_SYMBOL(enable_kernel_altivec);
 
+extern void disable_kernel_altivec(void)
+{
+	WARN_ON_ONCE(!this_cpu_read(in_kernel_simd));
+	this_cpu_write(in_kernel_simd, 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.
@@ -371,7 +404,8 @@ static bool interrupted_user_mode(void)
 bool irq_simd_usable(void)
 {
 	return !in_interrupt() ||
-		interrupted_user_mode();
+		interrupted_user_mode() ||
+		interrupted_kernel_simd_idle();
 }
 EXPORT_SYMBOL(irq_simd_usable);
 
@@ -411,7 +445,9 @@ void enable_kernel_vsx(void)
 {
 	unsigned long cpumsr;
 
-	WARN_ON(preemptible());
+	WARN_ON_ONCE(preemptible());
+	WARN_ON_ONCE(this_cpu_read(in_kernel_simd));
+	this_cpu_write(in_kernel_simd, true);
 
 	cpumsr = msr_check_and_set(MSR_FP|MSR_VEC|MSR_VSX);
 
@@ -433,6 +469,14 @@ void enable_kernel_vsx(void)
 }
 EXPORT_SYMBOL(enable_kernel_vsx);
 
+void disable_kernel_vsx(void)
+{
+	WARN_ON_ONCE(!this_cpu_read(in_kernel_simd));
+	this_cpu_write(in_kernel_simd, 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