x86/process: Move the buffer clearing before MONITOR

Commit 8e786a85c0a3c0fffae6244733fb576eeabd9dec upstream.

Move the VERW clearing before the MONITOR so that VERW doesn't disarm it
and the machine never enters C1.

Original idea by Kim Phillips <[email protected]>.

Suggested-by: Andrew Cooper <[email protected]>
Signed-off-by: Borislav Petkov (AMD) <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>

Author: bp3tk0v

arch/x86/include/asm/mwait.h

@@ -44,8 +44,6 @@ static inline void __monitorx(const void *eax, unsigned long ecx,
 
 static inline void __mwait(unsigned long eax, unsigned long ecx)
 {
-	x86_idle_clear_cpu_buffers();
-
 	/* "mwait %eax, %ecx;" */
 	asm volatile(".byte 0x0f, 0x01, 0xc9;"
 		     :: "a" (eax), "c" (ecx));
@@ -89,7 +87,6 @@ static inline void __mwaitx(unsigned long eax, unsigned long ebx,
 
 static inline void __sti_mwait(unsigned long eax, unsigned long ecx)
 {
-	x86_idle_clear_cpu_buffers();
 
 	/* "mwait %eax, %ecx;" */
 	asm volatile("sti; .byte 0x0f, 0x01, 0xc9;"
@@ -108,6 +105,11 @@ static inline void __sti_mwait(unsigned long eax, unsigned long ecx)
  */
 static inline void mwait_idle_with_hints(unsigned long eax, unsigned long ecx)
 {
+	if (need_resched())
+		return;
+
+	x86_idle_clear_cpu_buffers();
+
 	if (static_cpu_has_bug(X86_BUG_MONITOR) || !current_set_polling_and_test()) {
 		if (static_cpu_has_bug(X86_BUG_CLFLUSH_MONITOR)) {
 			mb();
@@ -116,9 +118,13 @@ static inline void mwait_idle_with_hints(unsigned long eax, unsigned long ecx)
 		}
 
 		__monitor((void *)&current_thread_info()->flags, 0, 0);
-		if (!need_resched())
-			__mwait(eax, ecx);
+		if (need_resched())
+			goto out;
+
+		__mwait(eax, ecx);
 	}
+
+out:
 	current_clr_polling();
 }
 

arch/x86/kernel/process.c

@@ -887,6 +887,11 @@ static int prefer_mwait_c1_over_halt(const struct cpuinfo_x86 *c)
  */
 static __cpuidle void mwait_idle(void)
 {
+	if (need_resched())
+		return;
+
+	x86_idle_clear_cpu_buffers();
+
 	if (!current_set_polling_and_test()) {
 		if (this_cpu_has(X86_BUG_CLFLUSH_MONITOR)) {
 			mb(); /* quirk */
@@ -895,13 +900,17 @@ static __cpuidle void mwait_idle(void)
 		}
 
 		__monitor((void *)&current_thread_info()->flags, 0, 0);
-		if (!need_resched())
-			__sti_mwait(0, 0);
-		else
+		if (need_resched()) {
 			raw_local_irq_enable();
+			goto out;
+		}
+
+		__sti_mwait(0, 0);
 	} else {
 		raw_local_irq_enable();
 	}
+
+out:
 	__current_clr_polling();
 }