[Xen-devel] [PATCH RFC V8 16/17] kvm : Paravirtual ticketlocks support for linux guests running on KVM hypervisor

[Raghavendra K T <raghavendra.kt@xxxxxxxxxxxxxxxxxx>]

From: Srivatsa Vaddagiri <vatsa@xxxxxxxxxxxxxxxxxx>

During smp_boot_cpus  paravirtualied KVM guest detects if the hypervisor has
required feature (KVM_FEATURE_PV_UNHALT) to support pv-ticketlocks. If so,
 support for pv-ticketlocks is registered via pv_lock_ops.

Use KVM_HC_KICK_CPU hypercall to wakeup waiting/halted vcpu.

Signed-off-by: Srivatsa Vaddagiri <vatsa@xxxxxxxxxxxxxxxxxx>
Signed-off-by: Suzuki Poulose <suzuki@xxxxxxxxxx>
Signed-off-by: Raghavendra K T <raghavendra.kt@xxxxxxxxxxxxxxxxxx>
---
 arch/x86/include/asm/kvm_para.h |   14 ++-
 arch/x86/kernel/kvm.c           |  256 +++++++++++++++++++++++++++++++++++++++
 2 files changed, 268 insertions(+), 2 deletions(-)
diff --git a/arch/x86/include/asm/kvm_para.h b/arch/x86/include/asm/kvm_para.h
index 5b647ea..77266d3 100644
--- a/arch/x86/include/asm/kvm_para.h
+++ b/arch/x86/include/asm/kvm_para.h
@@ -195,10 +195,20 @@ void kvm_async_pf_task_wait(u32 token);
 void kvm_async_pf_task_wake(u32 token);
 u32 kvm_read_and_reset_pf_reason(void);
 extern void kvm_disable_steal_time(void);
-#else
-#define kvm_guest_init() do { } while (0)
+
+#ifdef CONFIG_PARAVIRT_SPINLOCKS
+void __init kvm_spinlock_init(void);
+#else /* !CONFIG_PARAVIRT_SPINLOCKS */
+static inline void kvm_spinlock_init(void)
+{
+}
+#endif /* CONFIG_PARAVIRT_SPINLOCKS */
+
+#else /* CONFIG_KVM_GUEST */
+#define kvm_guest_init() do {} while (0)
 #define kvm_async_pf_task_wait(T) do {} while(0)
 #define kvm_async_pf_task_wake(T) do {} while(0)
+
 static inline u32 kvm_read_and_reset_pf_reason(void)
 {
        return 0;
diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
index b8ba6e4..7c46567 100644
--- a/arch/x86/kernel/kvm.c
+++ b/arch/x86/kernel/kvm.c
@@ -33,6 +33,7 @@
 #include <linux/sched.h>
 #include <linux/slab.h>
 #include <linux/kprobes.h>
+#include <linux/debugfs.h>
 #include <asm/timer.h>
 #include <asm/cpu.h>
 #include <asm/traps.h>
@@ -368,6 +369,7 @@ static void __init kvm_smp_prepare_boot_cpu(void)
 #endif
        kvm_guest_cpu_init();
        native_smp_prepare_boot_cpu();
+       kvm_spinlock_init();
 }
 
 static void __cpuinit kvm_guest_cpu_online(void *dummy)
@@ -450,3 +452,257 @@ static __init int activate_jump_labels(void)
        return 0;
 }
 arch_initcall(activate_jump_labels);
+
+/* Kick a cpu by its apicid. Used to wake up a halted vcpu */
+void kvm_kick_cpu(int cpu)
+{
+       int apicid;
+
+       apicid = per_cpu(x86_cpu_to_apicid, cpu);
+       kvm_hypercall1(KVM_HC_KICK_CPU, apicid);
+}
+
+#ifdef CONFIG_PARAVIRT_SPINLOCKS
+
+enum kvm_contention_stat {
+       TAKEN_SLOW,
+       TAKEN_SLOW_PICKUP,
+       RELEASED_SLOW,
+       RELEASED_SLOW_KICKED,
+       NR_CONTENTION_STATS
+};
+
+#ifdef CONFIG_KVM_DEBUG_FS
+#define HISTO_BUCKETS  30
+
+static struct kvm_spinlock_stats
+{
+       u32 contention_stats[NR_CONTENTION_STATS];
+       u32 histo_spin_blocked[HISTO_BUCKETS+1];
+       u64 time_blocked;
+} spinlock_stats;
+
+static u8 zero_stats;
+
+static inline void check_zero(void)
+{
+       u8 ret;
+       u8 old;
+
+       old = ACCESS_ONCE(zero_stats);
+       if (unlikely(old)) {
+               ret = cmpxchg(&zero_stats, old, 0);
+               /* This ensures only one fellow resets the stat */
+               if (ret == old)
+                       memset(&spinlock_stats, 0, sizeof(spinlock_stats));
+       }
+}
+
+static inline void add_stats(enum kvm_contention_stat var, u32 val)
+{
+       check_zero();
+       spinlock_stats.contention_stats[var] += val;
+}
+
+
+static inline u64 spin_time_start(void)
+{
+       return sched_clock();
+}
+
+static void __spin_time_accum(u64 delta, u32 *array)
+{
+       unsigned index;
+
+       index = ilog2(delta);
+       check_zero();
+
+       if (index < HISTO_BUCKETS)
+               array[index]++;
+       else
+               array[HISTO_BUCKETS]++;
+}
+
+static inline void spin_time_accum_blocked(u64 start)
+{
+       u32 delta;
+
+       delta = sched_clock() - start;
+       __spin_time_accum(delta, spinlock_stats.histo_spin_blocked);
+       spinlock_stats.time_blocked += delta;
+}
+
+static struct dentry *d_spin_debug;
+static struct dentry *d_kvm_debug;
+
+struct dentry *kvm_init_debugfs(void)
+{
+       d_kvm_debug = debugfs_create_dir("kvm", NULL);
+       if (!d_kvm_debug)
+               printk(KERN_WARNING "Could not create 'kvm' debugfs 
directory\n");
+
+       return d_kvm_debug;
+}
+
+static int __init kvm_spinlock_debugfs(void)
+{
+       struct dentry *d_kvm;
+
+       d_kvm = kvm_init_debugfs();
+       if (d_kvm == NULL)
+               return -ENOMEM;
+
+       d_spin_debug = debugfs_create_dir("spinlocks", d_kvm);
+
+       debugfs_create_u8("zero_stats", 0644, d_spin_debug, &zero_stats);
+
+       debugfs_create_u32("taken_slow", 0444, d_spin_debug,
+                  &spinlock_stats.contention_stats[TAKEN_SLOW]);
+       debugfs_create_u32("taken_slow_pickup", 0444, d_spin_debug,
+                  &spinlock_stats.contention_stats[TAKEN_SLOW_PICKUP]);
+
+       debugfs_create_u32("released_slow", 0444, d_spin_debug,
+                  &spinlock_stats.contention_stats[RELEASED_SLOW]);
+       debugfs_create_u32("released_slow_kicked", 0444, d_spin_debug,
+                  &spinlock_stats.contention_stats[RELEASED_SLOW_KICKED]);
+
+       debugfs_create_u64("time_blocked", 0444, d_spin_debug,
+                          &spinlock_stats.time_blocked);
+
+       debugfs_create_u32_array("histo_blocked", 0444, d_spin_debug,
+                    spinlock_stats.histo_spin_blocked, HISTO_BUCKETS + 1);
+
+       return 0;
+}
+fs_initcall(kvm_spinlock_debugfs);
+#else  /* !CONFIG_KVM_DEBUG_FS */
+#define TIMEOUT                        (1 << 10)
+static inline void add_stats(enum kvm_contention_stat var, u32 val)
+{
+}
+
+static inline u64 spin_time_start(void)
+{
+       return 0;
+}
+
+static inline void spin_time_accum_blocked(u64 start)
+{
+}
+#endif  /* CONFIG_KVM_DEBUG_FS */
+
+struct kvm_lock_waiting {
+       struct arch_spinlock *lock;
+       __ticket_t want;
+};
+
+/* cpus 'waiting' on a spinlock to become available */
+static cpumask_t waiting_cpus;
+
+/* Track spinlock on which a cpu is waiting */
+static DEFINE_PER_CPU(struct kvm_lock_waiting, lock_waiting);
+
+static void kvm_lock_spinning(struct arch_spinlock *lock, __ticket_t want)
+{
+       struct kvm_lock_waiting *w;
+       int cpu;
+       u64 start;
+       unsigned long flags;
+
+       w = &__get_cpu_var(lock_waiting);
+       cpu = smp_processor_id();
+       start = spin_time_start();
+
+       /*
+        * Make sure an interrupt handler can't upset things in a
+        * partially setup state.
+        */
+       local_irq_save(flags);
+
+       /*
+        * The ordering protocol on this is that the "lock" pointer
+        * may only be set non-NULL if the "want" ticket is correct.
+        * If we're updating "want", we must first clear "lock".
+        */
+       w->lock = NULL;
+       smp_wmb();
+       w->want = want;
+       smp_wmb();
+       w->lock = lock;
+
+       add_stats(TAKEN_SLOW, 1);
+
+       /*
+        * This uses set_bit, which is atomic but we should not rely on its
+        * reordering gurantees. So barrier is needed after this call.
+        */
+       cpumask_set_cpu(cpu, &waiting_cpus);
+
+       barrier();
+
+       /*
+        * Mark entry to slowpath before doing the pickup test to make
+        * sure we don't deadlock with an unlocker.
+        */
+       __ticket_enter_slowpath(lock);
+
+       /*
+        * check again make sure it didn't become free while
+        * we weren't looking.
+        */
+       if (ACCESS_ONCE(lock->tickets.head) == want) {
+               add_stats(TAKEN_SLOW_PICKUP, 1);
+               goto out;
+       }
+
+       /* Allow interrupts while blocked */
+       local_irq_restore(flags);
+
+       /* halt until it's our turn and kicked. */
+       halt();
+
+       local_irq_save(flags);
+out:
+       cpumask_clear_cpu(cpu, &waiting_cpus);
+       w->lock = NULL;
+       local_irq_restore(flags);
+       spin_time_accum_blocked(start);
+}
+PV_CALLEE_SAVE_REGS_THUNK(kvm_lock_spinning);
+
+/* Kick vcpu waiting on @lock->head to reach value @ticket */
+static void kvm_unlock_kick(struct arch_spinlock *lock, __ticket_t ticket)
+{
+       int cpu;
+
+       add_stats(RELEASED_SLOW, 1);
+       for_each_cpu(cpu, &waiting_cpus) {
+               const struct kvm_lock_waiting *w = &per_cpu(lock_waiting, cpu);
+               if (ACCESS_ONCE(w->lock) == lock &&
+                   ACCESS_ONCE(w->want) == ticket) {
+                       add_stats(RELEASED_SLOW_KICKED, 1);
+                       kvm_kick_cpu(cpu);
+                       break;
+               }
+       }
+}
+
+/*
+ * Setup pv_lock_ops to exploit KVM_FEATURE_PV_UNHALT if present.
+ */
+void __init kvm_spinlock_init(void)
+{
+       if (!kvm_para_available())
+               return;
+       /* Does host kernel support KVM_FEATURE_PV_UNHALT? */
+       if (!kvm_para_has_feature(KVM_FEATURE_PV_UNHALT))
+               return;
+
+       printk(KERN_INFO"KVM setup paravirtual spinlock\n");
+
+       static_key_slow_inc(&paravirt_ticketlocks_enabled);
+
+       pv_lock_ops.lock_spinning = PV_CALLEE_SAVE(kvm_lock_spinning);
+       pv_lock_ops.unlock_kick = kvm_unlock_kick;
+}
+#endif /* CONFIG_PARAVIRT_SPINLOCKS */


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