selftests/x86: Use __builtin_ia32_read/writeeflags

The asm to read and write EFLAGS from userspace is horrible.  The
compiler builtins are now available on all supported compilers, so
use them instead.

(The compiler builtins are also unnecessarily ugly, but that's a
 more manageable level of ugliness.)
Signed-off-by: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lkml.kernel.org/r/aee4b1cdfc56083eb779ce927b7d3459aad2af76.1604346818.git.luto@kernel.org

tools/testing/selftests/x86/helpers.h

@@ -6,36 +6,20 @@
 
 static inline unsigned long get_eflags(void)
 {
-	unsigned long eflags;
-
-	asm volatile (
 #ifdef __x86_64__
-		"subq $128, %%rsp\n\t"
-		"pushfq\n\t"
-		"popq %0\n\t"
-		"addq $128, %%rsp"
+	return __builtin_ia32_readeflags_u64();
 #else
-		"pushfl\n\t"
-		"popl %0"
+	return __builtin_ia32_readeflags_u32();
 #endif
-		: "=r" (eflags) :: "memory");
-
-	return eflags;
 }
 
 static inline void set_eflags(unsigned long eflags)
 {
-	asm volatile (
 #ifdef __x86_64__
-		"subq $128, %%rsp\n\t"
-		"pushq %0\n\t"
-		"popfq\n\t"
-		"addq $128, %%rsp"
+	__builtin_ia32_writeeflags_u64(eflags);
 #else
-		"pushl %0\n\t"
-		"popfl"
+	__builtin_ia32_writeeflags_u32(eflags);
 #endif
-		:: "r" (eflags) : "flags", "memory");
 }
 
 #endif /* __SELFTESTS_X86_HELPERS_H */