s390/fpu: make kernel fpu context preemptible

Make the kernel fpu context preemptible. Add another fpu structure to the
thread_struct, and use it to save and restore the kernel fpu context if its
task uses fpu registers when it is preempted.
Reviewed-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>

arch/s390/include/asm/fpu.h

@@ -47,6 +47,7 @@
 #include <linux/processor.h>
 #include <linux/preempt.h>
 #include <linux/string.h>
+#include <linux/sched.h>
 #include <asm/sigcontext.h>
 #include <asm/fpu-types.h>
 #include <asm/fpu-insn.h>
@@ -82,13 +83,6 @@ enum {
 #define KERNEL_VXR		(KERNEL_VXR_LOW	   | KERNEL_VXR_HIGH)
 #define KERNEL_FPR		(KERNEL_FPC	   | KERNEL_VXR_LOW)
 
-/*
- * 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, int flags);
 void __kernel_fpu_end(struct kernel_fpu *state, int flags);
 
@@ -146,8 +140,7 @@ static __always_inline void load_fp_regs(freg_t *fprs)
 
 static inline void kernel_fpu_begin(struct kernel_fpu *state, int flags)
 {
-	preempt_disable();
-	state->mask = S390_lowcore.fpu_flags;
+	state->mask = READ_ONCE(current->thread.kfpu_flags);
 	if (!test_thread_flag(TIF_FPU)) {
 		/* Save user space FPU state and register contents */
 		save_user_fpu_regs();
@@ -155,17 +148,42 @@ static inline void kernel_fpu_begin(struct kernel_fpu *state, int flags)
 		/* Save FPU/vector register in-use by the kernel */
 		__kernel_fpu_begin(state, flags);
 	}
-	S390_lowcore.fpu_flags |= flags;
+	__atomic_or(flags, &current->thread.kfpu_flags);
 }
 
 static inline void kernel_fpu_end(struct kernel_fpu *state, int flags)
 {
-	S390_lowcore.fpu_flags = state->mask;
+	WRITE_ONCE(current->thread.kfpu_flags, state->mask);
 	if (state->mask & flags) {
 		/* Restore FPU/vector register in-use by the kernel */
 		__kernel_fpu_end(state, flags);
 	}
-	preempt_enable();
+}
+
+static inline void save_kernel_fpu_regs(struct thread_struct *thread)
+{
+	struct fpu *state = &thread->kfpu;
+
+	if (!thread->kfpu_flags)
+		return;
+	fpu_stfpc(&state->fpc);
+	if (likely(cpu_has_vx()))
+		save_vx_regs(state->vxrs);
+	else
+		save_fp_regs(state->fprs);
+}
+
+static inline void restore_kernel_fpu_regs(struct thread_struct *thread)
+{
+	struct fpu *state = &thread->kfpu;
+
+	if (!thread->kfpu_flags)
+		return;
+	fpu_lfpc(&state->fpc);
+	if (likely(cpu_has_vx()))
+		load_vx_regs(state->vxrs);
+	else
+		load_fp_regs(state->fprs);
 }
 
 static inline void convert_vx_to_fp(freg_t *fprs, __vector128 *vxrs)

arch/s390/include/asm/lowcore.h

@@ -157,7 +157,7 @@ struct lowcore {
 	__s32	preempt_count;			/* 0x03a8 */
 	__u32	spinlock_lockval;		/* 0x03ac */
 	__u32	spinlock_index;			/* 0x03b0 */
-	__u32	fpu_flags;			/* 0x03b4 */
+	__u8	pad_0x03b4[0x03b8-0x03b4];	/* 0x03b4 */
 	__u64	percpu_offset;			/* 0x03b8 */
 	__u8	pad_0x03c0[0x03c8-0x03c0];	/* 0x03c0 */
 	__u64	machine_flags;			/* 0x03c8 */

arch/s390/include/asm/processor.h

@@ -166,6 +166,7 @@ struct thread_struct {
 	unsigned int gmap_write_flag;		/* gmap fault write indication */
 	unsigned int gmap_int_code;		/* int code of last gmap fault */
 	unsigned int gmap_pfault;		/* signal of a pending guest pfault */
+	int kfpu_flags;				/* kernel fpu flags */
 
 	/* Per-thread information related to debugging */
 	struct per_regs per_user;		/* User specified PER registers */
@@ -182,6 +183,7 @@ struct thread_struct {
 	struct gs_cb *gs_bc_cb;			/* Broadcast guarded storage cb */
 	struct pgm_tdb trap_tdb;		/* Transaction abort diagnose block */
 	struct fpu ufpu;			/* User FP and VX register save area */
+	struct fpu kfpu;			/* Kernel FP and VX register save area */
 };
 
 /* Flag to disable transactions. */

arch/s390/kernel/process.c

@@ -95,6 +95,7 @@ int arch_dup_task_struct(struct task_struct *dst, struct task_struct *src)
 
 	*dst = *src;
 	dst->thread.ufpu.regs = dst->thread.ufpu.fprs;
+	dst->thread.kfpu_flags = 0;
 
 	/*
 	 * Don't transfer over the runtime instrumentation or the guarded
@@ -197,10 +198,12 @@ void execve_tail(void)
 struct task_struct *__switch_to(struct task_struct *prev, struct task_struct *next)
 {
 	save_user_fpu_regs();
+	save_kernel_fpu_regs(&prev->thread);
 	save_access_regs(&prev->thread.acrs[0]);
 	save_ri_cb(prev->thread.ri_cb);
 	save_gs_cb(prev->thread.gs_cb);
 	update_cr_regs(next);
+	restore_kernel_fpu_regs(&next->thread);
 	restore_access_regs(&next->thread.acrs[0]);
 	restore_ri_cb(next->thread.ri_cb, prev->thread.ri_cb);
 	restore_gs_cb(next->thread.gs_cb);