openrisc: Move FPU state out of pt_regs

My original, naive, FPU support patch had the FPCSR register stored
during both the *mode switch* and *context switch*.  This is wasteful.

Also, the original patches did not save the FPU state when handling
signals during the system call fast path.

We fix this by moving the FPCSR state to thread_struct in task_struct.
We also introduce new helper functions save_fpu and restore_fpu which
can be used to sync the FPU with thread_struct.  These functions are now
called when needed:

 - Setting up and restoring sigcontext when handling signals
 - Before and after __switch_to during context switches
 - When handling FPU exceptions
 - When reading and writing FPU register sets

In the future we can further optimize this by doing lazy FPU save and
restore.  For example, FPU sync is not needed when switching to and from
kernel threads (x86 does this).  FPU save and restore does not need to
be done two times if we have both rescheduling and signal work to do.
However, since OpenRISC FPU state is a single register, I leave these
optimizations for future consideration.

Signed-off-by: Stafford Horne <shorne@gmail.com>

3 files changed, 24 insertions, 2 deletions

diff --git a/arch/openrisc/include/asm/fpu.h b/arch/openrisc/include/asm/fpu.h
new file mode 100644
index 000000000000..57bc44d80d53
--- /dev/null
+++ b/arch/openrisc/include/asm/fpu.h
@@ -0,0 +1,22 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef __ASM_OPENRISC_FPU_H
+#define __ASM_OPENRISC_FPU_H
+
+struct task_struct;
+
+#ifdef CONFIG_FPU
+static inline void save_fpu(struct task_struct *task)
+{
+	task->thread.fpcsr = mfspr(SPR_FPCSR);
+}
+
+static inline void restore_fpu(struct task_struct *task)
+{
+	mtspr(SPR_FPCSR, task->thread.fpcsr);
+}
+#else
+#define save_fpu(tsk)			do { } while (0)
+#define restore_fpu(tsk)		do { } while (0)
+#endif
+
+#endif /* __ASM_OPENRISC_FPU_H */
diff --git a/arch/openrisc/include/asm/processor.h b/arch/openrisc/include/asm/processor.h
index 3b736e74e6ed..e05d1b59e24e 100644
--- a/arch/openrisc/include/asm/processor.h
+++ b/arch/openrisc/include/asm/processor.h
@@ -44,6 +44,7 @@
 struct task_struct;
 
 struct thread_struct {
+	long fpcsr;		/* Floating point control status register. */
 };
 
 /*
diff --git a/arch/openrisc/include/asm/ptrace.h b/arch/openrisc/include/asm/ptrace.h
index 375147ff71fc..1da3e66292e2 100644
--- a/arch/openrisc/include/asm/ptrace.h
+++ b/arch/openrisc/include/asm/ptrace.h
@@ -59,7 +59,7 @@ struct pt_regs {
 	 * -1 for all other exceptions.
 	 */
 	long  orig_gpr11;	/* For restarting system calls */
-	long fpcsr;		/* Floating point control status register. */
+	long dummy;		/* Cheap alignment fix */
 	long dummy2;		/* Cheap alignment fix */
 };
 
@@ -115,6 +115,5 @@ static inline long regs_return_value(struct pt_regs *regs)
 #define PT_GPR31      124
 #define PT_PC	      128
 #define PT_ORIG_GPR11 132
-#define PT_FPCSR      136
 
 #endif /* __ASM_OPENRISC_PTRACE_H */