s390/vx: add support functions for in-kernel FPU use

Introduce the kernel_fpu_begin() and kernel_fpu_end() function
to enclose any in-kernel use of FPU instructions and registers.
In enclosed sections, you can perform floating-point or vector
(SIMD) computations.  The functions take care of saving and
restoring FPU register contents and controls.

For usage details, see the guidelines in arch/s390/include/asm/fpu/api.h

Signed-off-by: Hendrik Brueckner <brueckner@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

2 files changed, 85 insertions, 0 deletions

diff --git a/arch/s390/include/asm/fpu/api.h b/arch/s390/include/asm/fpu/api.h
index 5e04f3cbd320..78ba3ddb9e18 100644
--- a/arch/s390/include/asm/fpu/api.h
+++ b/arch/s390/include/asm/fpu/api.h
@@ -1,6 +1,41 @@
 /*
  * In-kernel FPU support functions
  *
+ *
+ * Consider these guidelines before using in-kernel FPU functions:
+ *
+ *  1. Use kernel_fpu_begin() and kernel_fpu_end() to enclose all in-kernel
+ *     use of floating-point or vector registers and instructions.
+ *
+ *  2. For kernel_fpu_begin(), specify the vector register range you want to
+ *     use with the KERNEL_VXR_* constants. Consider these usage guidelines:
+ *
+ *     a) If your function typically runs in process-context, use the lower
+ *	  half of the vector registers, for example, specify KERNEL_VXR_LOW.
+ *     b) If your function typically runs in soft-irq or hard-irq context,
+ *	  prefer using the upper half of the vector registers, for example,
+ *	  specify KERNEL_VXR_HIGH.
+ *
+ *     If you adhere to these guidelines, an interrupted process context
+ *     does not require to save and restore vector registers because of
+ *     disjoint register ranges.
+ *
+ *     Also note that the __kernel_fpu_begin()/__kernel_fpu_end() functions
+ *     includes logic to save and restore up to 16 vector registers at once.
+ *
+ *  3. You can nest kernel_fpu_begin()/kernel_fpu_end() by using different
+ *     struct kernel_fpu states.  Vector registers that are in use by outer
+ *     levels are saved and restored.  You can minimize the save and restore
+ *     effort by choosing disjoint vector register ranges.
+ *
+ *  5. To use vector floating-point instructions, specify the KERNEL_FPC
+ *     flag to save and restore floating-point controls in addition to any
+ *     vector register range.
+ *
+ *  6. To use floating-point registers and instructions only, specify the
+ *     KERNEL_FPR flag.  This flag triggers a save and restore of vector
+ *     registers V0 to V15 and floating-point controls.
+ *
  * Copyright IBM Corp. 2015
  * Author(s): Hendrik Brueckner <brueckner@linux.vnet.ibm.com>
  */
@@ -8,6 +43,8 @@
 #ifndef _ASM_S390_FPU_API_H
 #define _ASM_S390_FPU_API_H
 
+#include <linux/preempt.h>
+
 void save_fpu_regs(void);
 
 static inline int test_fp_ctl(u32 fpc)
@@ -27,4 +64,42 @@ static inline int test_fp_ctl(u32 fpc)
 	return rc;
 }
 
+#define KERNEL_VXR_V0V7		1
+#define KERNEL_VXR_V8V15	2
+#define KERNEL_VXR_V16V23	4
+#define KERNEL_VXR_V24V31	8
+#define KERNEL_FPR		16
+#define KERNEL_FPC		256
+
+#define KERNEL_VXR_LOW		(KERNEL_VXR_V0V7|KERNEL_VXR_V8V15)
+#define KERNEL_VXR_MID		(KERNEL_VXR_V8V15|KERNEL_VXR_V16V23)
+#define KERNEL_VXR_HIGH		(KERNEL_VXR_V16V23|KERNEL_VXR_V24V31)
+
+#define KERNEL_FPU_MASK		(KERNEL_VXR_LOW|KERNEL_VXR_HIGH|KERNEL_FPR)
+
+struct kernel_fpu;
+
+/*
+ * Note the functions below must be called with preemption disabled.
+ * Do not enable preemption before calling __kernel_fpu_end() to prevent
+ * an corruption of an existing kernel FPU state.
+ *
+ * Prefer using the kernel_fpu_begin()/kernel_fpu_end() pair of functions.
+ */
+void __kernel_fpu_begin(struct kernel_fpu *state, u32 flags);
+void __kernel_fpu_end(struct kernel_fpu *state);
+
+
+static inline void kernel_fpu_begin(struct kernel_fpu *state, u32 flags)
+{
+	preempt_disable();
+	__kernel_fpu_begin(state, flags);
+}
+
+static inline void kernel_fpu_end(struct kernel_fpu *state)
+{
+	__kernel_fpu_end(state);
+	preempt_enable();
+}
+
 #endif /* _ASM_S390_FPU_API_H */
diff --git a/arch/s390/include/asm/fpu/types.h b/arch/s390/include/asm/fpu/types.h
index fe937c9b6471..bce255ead72b 100644
--- a/arch/s390/include/asm/fpu/types.h
+++ b/arch/s390/include/asm/fpu/types.h
@@ -24,4 +24,14 @@ struct fpu {
 /* VX array structure for address operand constraints in inline assemblies */
 struct vx_array { __vector128 _[__NUM_VXRS]; };
 
+/* In-kernel FPU state structure */
+struct kernel_fpu {
+	u32	    mask;
+	u32	    fpc;
+	union {
+		freg_t fprs[__NUM_FPRS];
+		__vector128 vxrs[__NUM_VXRS];
+	};
+};
+
 #endif /* _ASM_S390_FPU_TYPES_H */