x86/fpu: Add an __fpregs_load_activate() internal helper

Add a helper function that ensures the floating point registers for the
current task are active. Use with preemption disabled.

While at it, add fpregs_lock/unlock() helpers too, to be used in later
patches.

 [ bp: Add a comment about its intended usage. ]
Signed-off-by: Rik van Riel <riel@surriel.com>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Borislav Petkov <bp@suse.de>
Reviewed-by: Dave Hansen <dave.hansen@intel.com>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Aubrey Li <aubrey.li@intel.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Jann Horn <jannh@google.com>
Cc: "Jason A. Donenfeld" <Jason@zx2c4.com>
Cc: kvm ML <kvm@vger.kernel.org>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Radim Krčmář <rkrcmar@redhat.com>
Cc: Rik van Riel <riel@surriel.com>
Cc: x86-ml <x86@kernel.org>
Link: https://lkml.kernel.org/r/20190403164156.19645-10-bigeasy@linutronix.de

arch/x86/include/asm/fpu/api.h

@@ -10,6 +10,7 @@
 
 #ifndef _ASM_X86_FPU_API_H
 #define _ASM_X86_FPU_API_H
+#include <linux/preempt.h>
 
 /*
  * Use kernel_fpu_begin/end() if you intend to use FPU in kernel context. It
@@ -22,6 +23,16 @@ extern void kernel_fpu_begin(void);
 extern void kernel_fpu_end(void);
 extern bool irq_fpu_usable(void);
 
+static inline void fpregs_lock(void)
+{
+	preempt_disable();
+}
+
+static inline void fpregs_unlock(void)
+{
+	preempt_enable();
+}
+
 /*
  * Query the presence of one or more xfeatures. Works on any legacy CPU as well.
  *

arch/x86/include/asm/fpu/internal.h

@@ -484,6 +484,18 @@ static inline void fpregs_activate(struct fpu *fpu)
 	trace_x86_fpu_regs_activated(fpu);
 }
 
+/*
+ * Internal helper, do not use directly. Use switch_fpu_return() instead.
+ */
+static inline void __fpregs_load_activate(struct fpu *fpu, int cpu)
+{
+	if (!fpregs_state_valid(fpu, cpu)) {
+		if (current->mm)
+			copy_kernel_to_fpregs(&fpu->state);
+		fpregs_activate(fpu);
+	}
+}
+
 /*
  * FPU state switching for scheduling.
  *
@@ -522,14 +534,8 @@ switch_fpu_prepare(struct fpu *old_fpu, int cpu)
  */
 static inline void switch_fpu_finish(struct fpu *new_fpu, int cpu)
 {
-	if (static_cpu_has(X86_FEATURE_FPU)) {
-		if (!fpregs_state_valid(new_fpu, cpu)) {
-			if (current->mm)
-				copy_kernel_to_fpregs(&new_fpu->state);
-		}
-
-		fpregs_activate(new_fpu);
-	}
+	if (static_cpu_has(X86_FEATURE_FPU))
+		__fpregs_load_activate(new_fpu, cpu);
 }
 
 /*