x86/alternatives, x86/fpu: Add 'alternatives_patched' debug flag and use it in xsave_state()

We'd like to use xsave_state() earlier, but its SYSTEM_BOOTING check
is too imprecise.

The real condition that xsave_state() would like to check is whether
alternative XSAVE instructions were patched into the kernel image
already.

Add such a (read-mostly) debug flag and use it in xsave_state().

Cc: Andy Lutomirski <[e-mail]>
Cc: Borislav Petkov <[e-mail]>
Cc: Dave Hansen <[e-mail]>
Cc: Fenghua Yu <[e-mail]>
Cc: H. Peter Anvin <[e-mail]>
Cc: Linus Torvalds <[e-mail]>
Cc: Oleg Nesterov <[e-mail]>
Cc: Peter Zijlstra <[e-mail]>
Cc: Thomas Gleixner <[e-mail]>
Signed-off-by: Ingo Molnar <[e-mail]>

arch/x86/include/asm/alternative.h

···
 	u8  padlen;		/* length of build-time padding */
 } __packed;
 
+/*
+ * Debug flag that can be tested to see whether alternative
+ * instructions were patched in already:
+ */
+extern int alternatives_patched;
+
 extern void alternative_instructions(void);
 extern void apply_alternatives(struct alt_instr *start, struct alt_instr *end);
 

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

···
 	u32 hmask = mask >> 32;
 	int err = 0;
 
-	WARN_ON(system_state == SYSTEM_BOOTING);
+	WARN_ON(!alternatives_patched);
 
 	/*
 	 * If xsaves is enabled, xsaves replaces xsaveopt because

arch/x86/kernel/alternative.c

···
 #include <asm/io.h>
 #include <asm/fixmap.h>
 
+int __read_mostly alternatives_patched;
+
+EXPORT_SYMBOL_GPL(alternatives_patched);
+
 #define MAX_PATCH_LEN (255-1)
 
 static int __initdata_or_module debug_alternative;
···
 	apply_paravirt(__parainstructions, __parainstructions_end);
 
 	restart_nmi();
+	alternatives_patched = 1;
 }
 
 /**