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Subject[PATCH 3/4] paravirt: add virt_spin_lock pvops function
Date
There are cases where a guest tries to switch spinlocks to bare metal
behavior (e.g. by setting "xen_nopvspin" boot parameter). Today this
has the downside of falling back to unfair test and set scheme for
qspinlocks due to virt_spin_lock() detecting the virtualized
environment.

Make virt_spin_lock() a paravirt operation in order to enable users
to select an explicit behavior like bare metal.

Signed-off-by: Juergen Gross <jgross@suse.com>
---
arch/x86/include/asm/paravirt.h | 5 ++++
arch/x86/include/asm/paravirt_types.h | 1 +
arch/x86/include/asm/qspinlock.h | 48 ++++++++++++++++++++++++-----------
arch/x86/kernel/paravirt-spinlocks.c | 14 ++++++++++
arch/x86/kernel/smpboot.c | 2 ++
5 files changed, 55 insertions(+), 15 deletions(-)

diff --git a/arch/x86/include/asm/paravirt.h b/arch/x86/include/asm/paravirt.h
index c25dd22f7c70..d9e954fb37df 100644
--- a/arch/x86/include/asm/paravirt.h
+++ b/arch/x86/include/asm/paravirt.h
@@ -725,6 +725,11 @@ static __always_inline bool pv_vcpu_is_preempted(long cpu)
return PVOP_CALLEE1(bool, pv_lock_ops.vcpu_is_preempted, cpu);
}

+static __always_inline bool pv_virt_spin_lock(struct qspinlock *lock)
+{
+ return PVOP_CALLEE1(bool, pv_lock_ops.virt_spin_lock, lock);
+}
+
#endif /* SMP && PARAVIRT_SPINLOCKS */

#ifdef CONFIG_X86_32
diff --git a/arch/x86/include/asm/paravirt_types.h b/arch/x86/include/asm/paravirt_types.h
index 19efefc0e27e..928f5e7953a7 100644
--- a/arch/x86/include/asm/paravirt_types.h
+++ b/arch/x86/include/asm/paravirt_types.h
@@ -319,6 +319,7 @@ struct pv_lock_ops {
void (*kick)(int cpu);

struct paravirt_callee_save vcpu_is_preempted;
+ struct paravirt_callee_save virt_spin_lock;
} __no_randomize_layout;

/* This contains all the paravirt structures: we get a convenient
diff --git a/arch/x86/include/asm/qspinlock.h b/arch/x86/include/asm/qspinlock.h
index 48a706f641f2..fbd98896385c 100644
--- a/arch/x86/include/asm/qspinlock.h
+++ b/arch/x86/include/asm/qspinlock.h
@@ -17,6 +17,25 @@ static inline void native_queued_spin_unlock(struct qspinlock *lock)
smp_store_release((u8 *)lock, 0);
}

+static inline bool native_virt_spin_lock(struct qspinlock *lock)
+{
+ if (!static_cpu_has(X86_FEATURE_HYPERVISOR))
+ return false;
+
+ /*
+ * On hypervisors without PARAVIRT_SPINLOCKS support we fall
+ * back to a Test-and-Set spinlock, because fair locks have
+ * horrible lock 'holder' preemption issues.
+ */
+
+ do {
+ while (atomic_read(&lock->val) != 0)
+ cpu_relax();
+ } while (atomic_cmpxchg(&lock->val, 0, _Q_LOCKED_VAL) != 0);
+
+ return true;
+}
+
#ifdef CONFIG_PARAVIRT_SPINLOCKS
extern void native_queued_spin_lock_slowpath(struct qspinlock *lock, u32 val);
extern void __pv_init_lock_hash(void);
@@ -38,33 +57,32 @@ static inline bool vcpu_is_preempted(long cpu)
{
return pv_vcpu_is_preempted(cpu);
}
+
+void native_pv_lock_init(void) __init;
#else
static inline void queued_spin_unlock(struct qspinlock *lock)
{
native_queued_spin_unlock(lock);
}
+
+static inline void native_pv_lock_init(void)
+{
+}
#endif

#ifdef CONFIG_PARAVIRT
#define virt_spin_lock virt_spin_lock
+#ifdef CONFIG_PARAVIRT_SPINLOCKS
static inline bool virt_spin_lock(struct qspinlock *lock)
{
- if (!static_cpu_has(X86_FEATURE_HYPERVISOR))
- return false;
-
- /*
- * On hypervisors without PARAVIRT_SPINLOCKS support we fall
- * back to a Test-and-Set spinlock, because fair locks have
- * horrible lock 'holder' preemption issues.
- */
-
- do {
- while (atomic_read(&lock->val) != 0)
- cpu_relax();
- } while (atomic_cmpxchg(&lock->val, 0, _Q_LOCKED_VAL) != 0);
-
- return true;
+ return pv_virt_spin_lock(lock);
+}
+#else
+static inline bool virt_spin_lock(struct qspinlock *lock)
+{
+ return native_virt_spin_lock(lock);
}
+#endif /* CONFIG_PARAVIRT_SPINLOCKS */
#endif /* CONFIG_PARAVIRT */

#include <asm-generic/qspinlock.h>
diff --git a/arch/x86/kernel/paravirt-spinlocks.c b/arch/x86/kernel/paravirt-spinlocks.c
index 26e4bd92f309..1be187ef8a38 100644
--- a/arch/x86/kernel/paravirt-spinlocks.c
+++ b/arch/x86/kernel/paravirt-spinlocks.c
@@ -20,6 +20,12 @@ bool pv_is_native_spin_unlock(void)
__raw_callee_save___native_queued_spin_unlock;
}

+__visible bool __native_virt_spin_lock(struct qspinlock *lock)
+{
+ return native_virt_spin_lock(lock);
+}
+PV_CALLEE_SAVE_REGS_THUNK(__native_virt_spin_lock);
+
struct pv_lock_ops pv_lock_ops = {
#ifdef CONFIG_SMP
.queued_spin_lock_slowpath = native_queued_spin_lock_slowpath,
@@ -27,6 +33,14 @@ struct pv_lock_ops pv_lock_ops = {
.wait = paravirt_nop,
.kick = paravirt_nop,
.vcpu_is_preempted = __PV_IS_CALLEE_SAVE(_paravirt_false),
+ .virt_spin_lock = PV_CALLEE_SAVE(__native_virt_spin_lock),
#endif /* SMP */
};
EXPORT_SYMBOL(pv_lock_ops);
+
+void __init native_pv_lock_init(void)
+{
+ if (!static_cpu_has(X86_FEATURE_HYPERVISOR))
+ pv_lock_ops.virt_spin_lock =
+ __PV_IS_CALLEE_SAVE(_paravirt_false);
+}
diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c
index 54b9e89d4d6b..21500d3ba359 100644
--- a/arch/x86/kernel/smpboot.c
+++ b/arch/x86/kernel/smpboot.c
@@ -77,6 +77,7 @@
#include <asm/i8259.h>
#include <asm/realmode.h>
#include <asm/misc.h>
+#include <asm/qspinlock.h>

/* Number of siblings per CPU package */
int smp_num_siblings = 1;
@@ -1381,6 +1382,7 @@ void __init native_smp_prepare_boot_cpu(void)
/* already set me in cpu_online_mask in boot_cpu_init() */
cpumask_set_cpu(me, cpu_callout_mask);
cpu_set_state_online(me);
+ native_pv_lock_init();
}

void __init native_smp_cpus_done(unsigned int max_cpus)
--
2.12.3
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