x86/fpu: Split out the fpu__activate_fpstate_read() method

Currently fpu__activate_fpstate() is used for two distinct purposes:

  - read access by ptrace and core dumping, where in the core dumping
    case the current task's FPU state may be examined as well.

  - write access by ptrace, which modifies FPU registers and expects
    the modified registers to be reloaded on the next context switch.

Split out the reading side into fpu__activate_fpstate_read().

( Note that this is just a pure duplication of fpu__activate_fpstate()
  for the time being, we'll optimize the new function in the next patch. )

Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Bobby Powers <bobbypowers@gmail.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Fenghua Yu <fenghua.yu@intel.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Ingo Molnar <mingo@kernel.org>

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

@@ -22,6 +22,7 @@
  * High level FPU state handling functions:
  */
 extern void fpu__activate_curr(struct fpu *fpu);
+extern void fpu__activate_fpstate_read(struct fpu *fpu);
 extern void fpu__activate_fpstate(struct fpu *fpu);
 extern void fpu__save(struct fpu *fpu);
 extern void fpu__restore(struct fpu *fpu);

arch/x86/kernel/fpu/core.c

@@ -295,6 +295,35 @@ void fpu__activate_curr(struct fpu *fpu)
 }
 EXPORT_SYMBOL_GPL(fpu__activate_curr);
 
+/*
+ * This function must be called before we read a task's fpstate.
+ *
+ * If the task has not used the FPU before then initialize its
+ * fpstate.
+ *
+ * If the task has used the FPU before then save it.
+ */
+void fpu__activate_fpstate_read(struct fpu *fpu)
+{
+	/*
+	 * If fpregs are active (in the current CPU), then
+	 * copy them to the fpstate:
+	 */
+	if (fpu->fpregs_active) {
+		fpu__save(fpu);
+	} else {
+		if (fpu->fpstate_active) {
+			/* Invalidate any lazy state: */
+			fpu->last_cpu = -1;
+		} else {
+			fpstate_init(&fpu->state);
+
+			/* Safe to do for current and for stopped child tasks: */
+			fpu->fpstate_active = 1;
+		}
+	}
+}
+
 /*
  * This function must be called before we read or write a task's fpstate.
  *

arch/x86/kernel/fpu/regset.c

@@ -33,7 +33,7 @@ int xfpregs_get(struct task_struct *target, const struct user_regset *regset,
 	if (!cpu_has_fxsr)
 		return -ENODEV;
 
-	fpu__activate_fpstate(fpu);
+	fpu__activate_fpstate_read(fpu);
 	fpstate_sanitize_xstate(fpu);
 
 	return user_regset_copyout(&pos, &count, &kbuf, &ubuf,
@@ -82,7 +82,7 @@ int xstateregs_get(struct task_struct *target, const struct user_regset *regset,
 	if (!cpu_has_xsave)
 		return -ENODEV;
 
-	fpu__activate_fpstate(fpu);
+	fpu__activate_fpstate_read(fpu);
 
 	xsave = &fpu->state.xsave;
 
@@ -273,7 +273,7 @@ int fpregs_get(struct task_struct *target, const struct user_regset *regset,
 	struct fpu *fpu = &target->thread.fpu;
 	struct user_i387_ia32_struct env;
 
-	fpu__activate_fpstate(fpu);
+	fpu__activate_fpstate_read(fpu);
 
 	if (!static_cpu_has(X86_FEATURE_FPU))
 		return fpregs_soft_get(target, regset, pos, count, kbuf, ubuf);