x86/fpu: Don't abuse FPU in kernel threads if use_eager_fpu()

AFAICS, there is no reason why kernel threads should have FPU context
even if use_eager_fpu() == T. Now that interrupted_kernel_fpu_idle()
does not check __thread_has_fpu() in the use_eager_fpu() case, we
can remove the init_fpu() code from eager_fpu_init() and change
flush_thread() called by do_execve() to initialize FPU.

Note: of course, the change in flush_thread() is horrible and must be
cleanuped. We need the new helper, and flush_thread() should return the
error if init_fpu() fails.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Reviewed-by: Rik van Riel <riel@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Suresh Siddha <sbsiddha@gmail.com>
Cc: Andy Lutomirski <luto@amacapital.net>
Link: http://lkml.kernel.org/r/20150119185212.GD16427@redhat.com
Signed-off-by: Borislav Petkov <bp@suse.de>

arch/x86/kernel/process.c

@@ -130,6 +130,7 @@ void flush_thread(void)
 
 	flush_ptrace_hw_breakpoint(tsk);
 	memset(tsk->thread.tls_array, 0, sizeof(tsk->thread.tls_array));
+
 	drop_init_fpu(tsk);
 	/*
 	 * Free the FPU state for non xsave platforms. They get reallocated
@@ -137,6 +138,12 @@ void flush_thread(void)
 	 */
 	if (!use_eager_fpu())
 		free_thread_xstate(tsk);
+	else if (!used_math()) {
+		/* kthread execs. TODO: cleanup this horror. */
+		if (WARN_ON(init_fpu(current)))
+			force_sig(SIGKILL, current);
+		math_state_restore();
+	}
 }
 
 static void hard_disable_TSC(void)

arch/x86/kernel/xsave.c

@@ -688,7 +688,7 @@ void eager_fpu_init(void)
 {
 	static __refdata void (*boot_func)(void) = eager_fpu_init_bp;
 
-	clear_used_math();
+	WARN_ON(used_math());
 	current_thread_info()->status = 0;
 
 	if (eagerfpu == ENABLE)
@@ -703,17 +703,6 @@ void eager_fpu_init(void)
 		boot_func();
 		boot_func = NULL;
 	}
-
-	/*
-	 * This is same as math_state_restore(). But use_xsave() is
-	 * not yet patched to use math_state_restore().
-	 */
-	init_fpu(current);
-	__thread_fpu_begin(current);
-	if (cpu_has_xsave)
-		xrstor_state(init_xstate_buf, -1);
-	else
-		fxrstor_checking(&init_xstate_buf->i387);
 }
 
 /*