x86/asm/entry: Document and clean up the enable_sep_cpu() and syscall32_cpu_init() functions

Clean up the flow and document the functions a bit better.

Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: Alexei Starovoitov <ast@plumgrid.com>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Will Drewry <wad@chromium.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/kernel/cpu/common.c

@@ -960,37 +960,53 @@ static void identify_cpu(struct cpuinfo_x86 *c)
 }
 
 #ifdef CONFIG_X86_64
-#ifdef CONFIG_IA32_EMULATION
+# ifdef CONFIG_IA32_EMULATION
 /* May not be __init: called during resume */
 static void syscall32_cpu_init(void)
 {
-	/* Load these always in case some future AMD CPU supports
-	   SYSENTER from compat mode too. */
+	/*
+	 * Always load these, in case some future 64-bit CPU supports
+	 * SYSENTER from compat mode too:
+	 */
 	wrmsrl_safe(MSR_IA32_SYSENTER_CS, (u64)__KERNEL_CS);
 	wrmsrl_safe(MSR_IA32_SYSENTER_ESP, 0ULL);
 	wrmsrl_safe(MSR_IA32_SYSENTER_EIP, (u64)ia32_sysenter_target);
 
 	wrmsrl(MSR_CSTAR, ia32_cstar_target);
 }
-#endif		/* CONFIG_IA32_EMULATION */
-#endif		/* CONFIG_X86_64 */
+# endif
+#endif
 
+/*
+ * Set up the CPU state needed to execute SYSENTER/SYSEXIT instructions
+ * on 32-bit kernels:
+ */
 #ifdef CONFIG_X86_32
 void enable_sep_cpu(void)
 {
-	int cpu = get_cpu();
-	struct tss_struct *tss = &per_cpu(cpu_tss, cpu);
+	struct tss_struct *tss;
+	int cpu;
 
-	if (!boot_cpu_has(X86_FEATURE_SEP)) {
-		put_cpu();
-		return;
-	}
+	cpu = get_cpu();
+	tss = &per_cpu(cpu_tss, cpu);
+
+	if (!boot_cpu_has(X86_FEATURE_SEP))
+		goto out;
+
+	/*
+	 * The struct::SS1 and tss_struct::SP1 fields are not used by the hardware,
+	 * we cache the SYSENTER CS and ESP values there for easy access:
+	 */
 
 	tss->x86_tss.ss1 = __KERNEL_CS;
+	wrmsr(MSR_IA32_SYSENTER_CS, tss->x86_tss.ss1, 0);
+
 	tss->x86_tss.sp1 = (unsigned long)tss + offsetofend(struct tss_struct, SYSENTER_stack);
-	wrmsr(MSR_IA32_SYSENTER_CS, __KERNEL_CS, 0);
 	wrmsr(MSR_IA32_SYSENTER_ESP, tss->x86_tss.sp1, 0);
-	wrmsr(MSR_IA32_SYSENTER_EIP, (unsigned long) ia32_sysenter_target, 0);
+
+	wrmsr(MSR_IA32_SYSENTER_EIP, (unsigned long)ia32_sysenter_target, 0);
+
+out:
 	put_cpu();
 }
 #endif