[PATCH] x86: add config to disable PV spinlocks

[Jeremy Fitzhardinge]

Paravirtualized spinlocks seem to cause a 2-7% performance hit when
running a pvops kernel native.  Without them, the pvops kernel is
identical to a non-pvops kernel in performance.

[ Impact: reduce overhead of pvops when running native ]
Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@xxxxxxxxxx>

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index 5f50179..a99ed71 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -498,6 +498,17 @@ config PARAVIRT
 	  over full virtualization.  However, when run without a hypervisor
 	  the kernel is theoretically slower and slightly larger.
 
+config PARAVIRT_SPINLOCKS
+	bool "Enable paravirtualized spinlocks"
+	depends on PARAVIRT && SMP
+	default XEN
+	---help---
+	  Paravirtualized spinlocks allow a pvops backend to replace the
+	  spinlock implementation with something virtualization-friendly
+	  (for example, block the virtual CPU rather than spinning).
+	  Unfortunately the downside is an as-yet unexplained performance
+	  when running native.
+
 config PARAVIRT_CLOCK
 	bool
 	default n
diff --git a/arch/x86/include/asm/paravirt.h b/arch/x86/include/asm/paravirt.h
index 1fe5837..4fb37c8 100644
--- a/arch/x86/include/asm/paravirt.h
+++ b/arch/x86/include/asm/paravirt.h
@@ -1443,7 +1443,7 @@ u64 _paravirt_ident_64(u64);
 
 #define paravirt_nop	((void *)_paravirt_nop)
 
-#ifdef CONFIG_SMP
+#if defined(CONFIG_SMP) && defined(CONFIG_PARAVIRT_SPINLOCKS)
 
 static inline int __raw_spin_is_locked(struct raw_spinlock *lock)
 {
diff --git a/arch/x86/include/asm/spinlock.h b/arch/x86/include/asm/spinlock.h
index e5e6caf..b7e5db8 100644
--- a/arch/x86/include/asm/spinlock.h
+++ b/arch/x86/include/asm/spinlock.h
@@ -172,7 +172,7 @@ static inline int __ticket_spin_is_contended(raw_spinlock_t *lock)
 	return (((tmp >> TICKET_SHIFT) - tmp) & ((1 << TICKET_SHIFT) - 1)) > 1;
 }
 
-#ifndef CONFIG_PARAVIRT
+#ifndef CONFIG_PARAVIRT_SPINLOCKS
 
 static inline int __raw_spin_is_locked(raw_spinlock_t *lock)
 {
@@ -206,7 +206,7 @@ static __always_inline void __raw_spin_lock_flags(raw_spinlock_t *lock,
 	__raw_spin_lock(lock);
 }
 
-#endif
+#endif	/* CONFIG_PARAVIRT_SPINLOCKS */
 
 static inline void __raw_spin_unlock_wait(raw_spinlock_t *lock)
 {
diff --git a/arch/x86/kernel/Makefile b/arch/x86/kernel/Makefile
index 68a4ff6..4f78bd6 100644
--- a/arch/x86/kernel/Makefile
+++ b/arch/x86/kernel/Makefile
@@ -90,7 +90,8 @@ obj-$(CONFIG_DEBUG_NX_TEST)	+= test_nx.o
 obj-$(CONFIG_VMI)		+= vmi_32.o vmiclock_32.o
 obj-$(CONFIG_KVM_GUEST)		+= kvm.o
 obj-$(CONFIG_KVM_CLOCK)		+= kvmclock.o
-obj-$(CONFIG_PARAVIRT)		+= paravirt.o paravirt_patch_$(BITS).o paravirt-spinlocks.o
+obj-$(CONFIG_PARAVIRT)		+= paravirt.o paravirt_patch_$(BITS).o
+obj-$(CONFIG_PARAVIRT_SPINLOCKS)+= paravirt-spinlocks.o
 obj-$(CONFIG_PARAVIRT_CLOCK)	+= pvclock.o
 
 obj-$(CONFIG_PCSPKR_PLATFORM)	+= pcspeaker.o
diff --git a/arch/x86/kernel/paravirt.c b/arch/x86/kernel/paravirt.c
index aa34423..70ec9b9 100644
--- a/arch/x86/kernel/paravirt.c
+++ b/arch/x86/kernel/paravirt.c
@@ -134,7 +134,9 @@ static void *get_call_destination(u8 type)
 		.pv_irq_ops = pv_irq_ops,
 		.pv_apic_ops = pv_apic_ops,
 		.pv_mmu_ops = pv_mmu_ops,
+#ifdef CONFIG_PARAVIRT_SPINLOCKS
 		.pv_lock_ops = pv_lock_ops,
+#endif
 	};
 	return *((void **)&tmpl + type);
 }
diff --git a/arch/x86/xen/Makefile b/arch/x86/xen/Makefile
index 3b767d0..172438f 100644
--- a/arch/x86/xen/Makefile
+++ b/arch/x86/xen/Makefile
@@ -9,5 +9,6 @@ obj-y		:= enlighten.o setup.o multicalls.o mmu.o irq.o \
 			time.o xen-asm.o xen-asm_$(BITS).o \
 			grant-table.o suspend.o
 
-obj-$(CONFIG_SMP)		+= smp.o spinlock.o
-obj-$(CONFIG_XEN_DEBUG_FS)	+= debugfs.o
\ No newline at end of file
+obj-$(CONFIG_SMP)		+= smp.o
+obj-$(CONFIG_PARAVIRT_SPINLOCKS)+= spinlock.o
+obj-$(CONFIG_XEN_DEBUG_FS)	+= debugfs.o
diff --git a/arch/x86/xen/xen-ops.h b/arch/x86/xen/xen-ops.h
index 5c50a10..22494fd 100644
--- a/arch/x86/xen/xen-ops.h
+++ b/arch/x86/xen/xen-ops.h
@@ -61,15 +61,26 @@ void xen_setup_vcpu_info_placement(void);
 #ifdef CONFIG_SMP
 void xen_smp_init(void);
 
-void __init xen_init_spinlocks(void);
-__cpuinit void xen_init_lock_cpu(int cpu);
-void xen_uninit_lock_cpu(int cpu);
-
 extern cpumask_var_t xen_cpu_initialized_map;
 #else
 static inline void xen_smp_init(void) {}
 #endif
 
+#ifdef CONFIG_PARAVIRT_SPINLOCKS
+void __init xen_init_spinlocks(void);
+__cpuinit void xen_init_lock_cpu(int cpu);
+void xen_uninit_lock_cpu(int cpu);
+#else
+static inline void xen_init_spinlocks(void)
+{
+}
+static inline void xen_init_lock_cpu(int cpu)
+{
+}
+static inline void xen_uninit_lock_cpu(int cpu)
+{
+}
+#endif
 
 /* Declare an asm function, along with symbols needed to make it
    inlineable */


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