[Xen-devel] [PATCH 3/6] x86: introduce new pvops function clear_slowpath

[Juergen Gross <jgross@xxxxxxxx>]

To speed up paravirtualized spinlock handling when running on bare
metal introduce a new pvops function "clear_slowpath". This is a nop
when the kernel is running on bare metal.

As the clear_slowpath function is common for all users add a new
initialization function to set the pvops function pointer in order
to avoid spreading the knowledge which function to use.

Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
---
 arch/x86/include/asm/paravirt.h       |  7 +++++++
 arch/x86/include/asm/paravirt_types.h |  1 +
 arch/x86/include/asm/spinlock.h       | 18 ++++--------------
 arch/x86/kernel/kvm.c                 |  2 ++
 arch/x86/kernel/paravirt-spinlocks.c  | 22 ++++++++++++++++++++++
 arch/x86/xen/spinlock.c               |  2 ++
 6 files changed, 38 insertions(+), 14 deletions(-)

diff --git a/arch/x86/include/asm/paravirt.h b/arch/x86/include/asm/paravirt.h
index 8957810..318f077 100644
--- a/arch/x86/include/asm/paravirt.h
+++ b/arch/x86/include/asm/paravirt.h
@@ -724,6 +724,13 @@ static __always_inline void __ticket_unlock_kick(struct 
arch_spinlock *lock,
        PVOP_VCALL2(pv_lock_ops.unlock_kick, lock, ticket);
 }
 
+static __always_inline void __ticket_clear_slowpath(arch_spinlock_t *lock,
+                                                   __ticket_t head)
+{
+       PVOP_VCALL2(pv_lock_ops.clear_slowpath, lock, head);
+}
+
+void pv_lock_activate(void);
 #endif
 
 #ifdef CONFIG_X86_32
diff --git a/arch/x86/include/asm/paravirt_types.h 
b/arch/x86/include/asm/paravirt_types.h
index f7b0b5c..3432713 100644
--- a/arch/x86/include/asm/paravirt_types.h
+++ b/arch/x86/include/asm/paravirt_types.h
@@ -336,6 +336,7 @@ typedef u16 __ticket_t;
 struct pv_lock_ops {
        struct paravirt_callee_save lock_spinning;
        void (*unlock_kick)(struct arch_spinlock *lock, __ticket_t ticket);
+       void (*clear_slowpath)(arch_spinlock_t *lock, __ticket_t head);
 };
 
 /* This contains all the paravirt structures: we get a convenient
diff --git a/arch/x86/include/asm/spinlock.h b/arch/x86/include/asm/spinlock.h
index 268b9da..ab76c3e 100644
--- a/arch/x86/include/asm/spinlock.h
+++ b/arch/x86/include/asm/spinlock.h
@@ -60,26 +60,16 @@ static inline void __ticket_unlock_kick(arch_spinlock_t 
*lock,
 {
 }
 
+static inline void __ticket_clear_slowpath(arch_spinlock_t *lock,
+                                          __ticket_t head)
+{
+}
 #endif /* CONFIG_PARAVIRT_SPINLOCKS */
 static inline int  __tickets_equal(__ticket_t one, __ticket_t two)
 {
        return !((one ^ two) & ~TICKET_SLOWPATH_FLAG);
 }
 
-static inline void __ticket_clear_slowpath(arch_spinlock_t *lock,
-                                          __ticket_t head)
-{
-       arch_spinlock_t old, new;
-
-       old.tickets.head = head;
-       new.tickets.head = head & ~TICKET_SLOWPATH_FLAG;
-       old.tickets.tail = new.tickets.head + TICKET_LOCK_INC;
-       new.tickets.tail = old.tickets.tail;
-
-       /* try to clear slowpath flag when there are no contenders */
-       cmpxchg(&lock->head_tail, old.head_tail, new.head_tail);
-}
-
 static __always_inline int arch_spin_value_unlocked(arch_spinlock_t lock)
 {
        return __tickets_equal(lock.tickets.head, lock.tickets.tail);
diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
index 9435620..c3b4b43 100644
--- a/arch/x86/kernel/kvm.c
+++ b/arch/x86/kernel/kvm.c
@@ -830,6 +830,8 @@ void __init kvm_spinlock_init(void)
 
        pv_lock_ops.lock_spinning = PV_CALLEE_SAVE(kvm_lock_spinning);
        pv_lock_ops.unlock_kick = kvm_unlock_kick;
+
+       pv_lock_activate();
 }
 
 static __init int kvm_spinlock_init_jump(void)
diff --git a/arch/x86/kernel/paravirt-spinlocks.c 
b/arch/x86/kernel/paravirt-spinlocks.c
index bbb6c73..5ece813 100644
--- a/arch/x86/kernel/paravirt-spinlocks.c
+++ b/arch/x86/kernel/paravirt-spinlocks.c
@@ -8,10 +8,32 @@
 
 #include <asm/paravirt.h>
 
+#ifdef CONFIG_SMP
+static void pv_ticket_clear_slowpath(arch_spinlock_t *lock, __ticket_t head)
+{
+       arch_spinlock_t old, new;
+
+       old.tickets.head = head;
+       new.tickets.head = head & ~TICKET_SLOWPATH_FLAG;
+       old.tickets.tail = new.tickets.head + TICKET_LOCK_INC;
+       new.tickets.tail = old.tickets.tail;
+
+       /* try to clear slowpath flag when there are no contenders */
+       cmpxchg(&lock->head_tail, old.head_tail, new.head_tail);
+}
+
+void pv_lock_activate(void)
+{
+       pv_lock_ops.clear_slowpath = pv_ticket_clear_slowpath;
+}
+EXPORT_SYMBOL_GPL(pv_lock_activate);
+#endif
+
 struct pv_lock_ops pv_lock_ops = {
 #ifdef CONFIG_SMP
        .lock_spinning = __PV_IS_CALLEE_SAVE(paravirt_nop),
        .unlock_kick = paravirt_nop,
+       .clear_slowpath = paravirt_nop,
 #endif
 };
 EXPORT_SYMBOL(pv_lock_ops);
diff --git a/arch/x86/xen/spinlock.c b/arch/x86/xen/spinlock.c
index 956374c..988c895 100644
--- a/arch/x86/xen/spinlock.c
+++ b/arch/x86/xen/spinlock.c
@@ -282,6 +282,8 @@ void __init xen_init_spinlocks(void)
        printk(KERN_DEBUG "xen: PV spinlocks enabled\n");
        pv_lock_ops.lock_spinning = PV_CALLEE_SAVE(xen_lock_spinning);
        pv_lock_ops.unlock_kick = xen_unlock_kick;
+
+       pv_lock_activate();
 }
 
 /*
-- 
2.1.4


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