Subject: x86/fpu: use fnsave when fxsave is not available

kernel-2.6.32-754.3.5.el6 uses fxsave opcode unconditionally,
which causes swapper panic on early boot on i586.
Fix to use fnsave when use_fxsr() is false.

Upstream kernel.org code uses union init_fpstate.fsave to reset
fpu statn in drop_init_fpu() (fpu__clear() in kernel.org 4.4 code),
but linux-2.6.32-754.3.5.el6 only has 
	struct xsave_struct *init_xstate_buf;
so this patch adds and initializes
	struct fpu init_fpstate_fsave_buf;
explicitly for frstor resetting.
Only init_fpstate_fsave_buf.state->fsave is used, but to
initialize it using fpu_finit(), whole struct fpu is declared.

diff -up ./arch/x86/include/asm/fpu-internal.h.fxsave ./arch/x86/include/asm/fpu-internal.h
--- ./arch/x86/include/asm/fpu-internal.h.fxsave	2018-08-10 00:39:20.000000000 +0900
+++ ./arch/x86/include/asm/fpu-internal.h	2018-09-13 21:36:57.000000000 +0900
@@ -325,8 +325,10 @@ static inline void restore_init_xstate(v
 {
 	if (use_xsave())
 		xrstor_state(init_xstate_buf, -1);
-	else
+	else if (use_fxsr()) 
 		fxrstor_checking(&init_xstate_buf->i387);
+	else
+		frstor_checking(&init_fpstate_fsave_buf.state->fsave);
 }
 
 /*
@@ -463,8 +465,11 @@ static inline void __save_fpu(struct tas
 {
 	if (use_xsave())
 		xsave_state(&tsk->thread.xstate->xsave, -1);
-	else
+	else if (use_fxsr())
 		fpu_fxsave((struct fpu *)&tsk->thread.xstate);
+	else
+		asm volatile("fnsave %[fx]; fwait"
+			     : [fx] "=m" (tsk->thread.xstate->fsave));
 }
 
 /*
diff -up ./arch/x86/include/asm/xsave.h.fxsave ./arch/x86/include/asm/xsave.h
--- ./arch/x86/include/asm/xsave.h.fxsave	2018-08-10 00:39:20.000000000 +0900
+++ ./arch/x86/include/asm/xsave.h	2018-09-13 21:43:08.000000000 +0900
@@ -42,6 +42,7 @@ extern unsigned int xstate_size;
 extern u64 pcntxt_mask;
 extern u64 xstate_fx_sw_bytes[USER_XSTATE_FX_SW_WORDS];
 extern struct xsave_struct *init_xstate_buf;
+extern struct fpu init_fpstate_fsave_buf;
 
 extern void xsave_init(void);
 extern void update_regset_xstate_info(unsigned int size, u64 xstate_mask);
diff -up ./arch/x86/kernel/xsave.c.fxsave ./arch/x86/kernel/xsave.c
--- ./arch/x86/kernel/xsave.c.fxsave	2018-08-10 00:39:20.000000000 +0900
+++ ./arch/x86/kernel/xsave.c	2018-09-13 22:18:55.000000000 +0900
@@ -22,6 +22,7 @@ u64 pcntxt_mask;
  * Represents init state for the supported extended state.
  */
 struct xsave_struct *init_xstate_buf;
+struct fpu init_fpstate_fsave_buf;
 
 static struct _fpx_sw_bytes fx_sw_reserved, fx_sw_reserved_ia32;
 static unsigned int *xstate_offsets, *xstate_sizes, xstate_features;
@@ -489,6 +490,11 @@ static void __init setup_init_fpu_buf(vo
 					      __alignof__(struct xsave_struct));
 	fx_finit(&init_xstate_buf->i387);
 
+	/* Setup init_fpstate.fsave */
+	init_fpstate_fsave_buf.state = alloc_bootmem_align(sizeof(union thread_xstate),
+					                   __alignof__(union thread_xstate));
+	fpu_finit(&init_fpstate_fsave_buf);
+
 	if (!cpu_has_xsave)
 		return;
 
@@ -627,6 +633,8 @@ void __cpuinit eager_fpu_init(void)
 	__thread_fpu_begin(current);
 	if (cpu_has_xsave)
 		xrstor_state(init_xstate_buf, -1);
-	else
+	else if (cpu_has_fxsr)
 		fxrstor_checking(&init_xstate_buf->i387);
+	else
+		frstor_checking(&init_fpstate_fsave_buf.state->fsave);
 }