From 24a376d65177009a4dd8d846543c5dc69f5c4ced Mon Sep 17 00:00:00 2001
From: Peter Zijlstra <peterz@infradead.org>
Date: Thu, 1 Aug 2019 15:30:28 +0200
Subject: [PATCH] locking/qspinlock,x86: Clarify virt_spin_lock_key
Git-commit: 24a376d65177009a4dd8d846543c5dc69f5c4ced
Patch-mainline: v5.4-rc1
References: bsc#1149032

Add a few comments to clarify how this is supposed to work.

Reported-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Juergen Gross <jgross@suse.com>
Signed-off-by: Davidlohr Bueso <dbueso@suse.de>

---
 arch/x86/include/asm/qspinlock.h | 15 +++++++++++++++
 1 file changed, 15 insertions(+)

diff --git a/arch/x86/include/asm/qspinlock.h b/arch/x86/include/asm/qspinlock.h
index bd5ac6cc37db..444d6fd9a6d8 100644
--- a/arch/x86/include/asm/qspinlock.h
+++ b/arch/x86/include/asm/qspinlock.h
@@ -63,10 +63,25 @@ static inline bool vcpu_is_preempted(long cpu)
 #endif
 
 #ifdef CONFIG_PARAVIRT
+/*
+ * virt_spin_lock_key - enables (by default) the virt_spin_lock() hijack.
+ *
+ * Native (and PV wanting native due to vCPU pinning) should disable this key.
+ * It is done in this backwards fashion to only have a single direction change,
+ * which removes ordering between native_pv_spin_init() and HV setup.
+ */
 DECLARE_STATIC_KEY_TRUE(virt_spin_lock_key);
 
 void native_pv_lock_init(void) __init;
 
+/*
+ * Shortcut for the queued_spin_lock_slowpath() function that allows
+ * virt to hijack it.
+ *
+ * Returns:
+ *   true - lock has been negotiated, all done;
+ *   false - queued_spin_lock_slowpath() will do its thing.
+ */
 #define virt_spin_lock virt_spin_lock
 static inline bool virt_spin_lock(struct qspinlock *lock)
 {
-- 
2.16.4