x86/CPU/AMD: Always inline amd_clear_divider()

The routine is used on syscall exit and on non-AMD CPUs is guaranteed to
be empty.

It probably does not need to be a function call even on CPUs which do need the
mitigation.

  [ bp: Make sure it is always inlined so that noinstr marking works. ]

Signed-off-by: Mateusz Guzik <mjguzik@gmail.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Link: https://lore.kernel.org/r/20240613082637.659133-1-mjguzik@gmail.com

Author: mjguzik

arch/x86/include/asm/processor.h

@@ -692,7 +692,17 @@ static inline u32 per_cpu_l2c_id(unsigned int cpu)
 
 #ifdef CONFIG_CPU_SUP_AMD
 extern u32 amd_get_highest_perf(void);
-extern void amd_clear_divider(void);
+
+/*
+ * Issue a DIV 0/1 insn to clear any division data from previous DIV
+ * operations.
+ */
+static __always_inline void amd_clear_divider(void)
+{
+	asm volatile(ALTERNATIVE("", "div %2\n\t", X86_BUG_DIV0)
+		     :: "a" (0), "d" (0), "r" (1));
+}
+
 extern void amd_check_microcode(void);
 #else
 static inline u32 amd_get_highest_perf(void)		{ return 0; }

arch/x86/kernel/cpu/amd.c

@@ -1220,14 +1220,3 @@ void amd_check_microcode(void)
 
 	on_each_cpu(zenbleed_check_cpu, NULL, 1);
 }
-
-/*
- * Issue a DIV 0/1 insn to clear any division data from previous DIV
- * operations.
- */
-void noinstr amd_clear_divider(void)
-{
-	asm volatile(ALTERNATIVE("", "div %2\n\t", X86_BUG_DIV0)
-		     :: "a" (0), "d" (0), "r" (1));
-}
-EXPORT_SYMBOL_GPL(amd_clear_divider);