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    Subject[PATCH RFC V4 4/5] kvm : pv-ticketlocks support for linux guests running on KVM hypervisor
    Extends Linux guest running on KVM hypervisor to support pv-ticketlocks. 

    During smp_boot_cpus paravirtualied KVM guest detects if the hypervisor has
    required feature (KVM_FEATURE_PVLOCK_KICK) 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@linux.vnet.ibm.com>
    Signed-off-by: Suzuki Poulose <suzuki@in.ibm.com>
    Signed-off-by: Raghavendra K T <raghavendra.kt@linux.vnet.ibm.com>
    ---
    diff --git a/arch/x86/include/asm/kvm_para.h b/arch/x86/include/asm/kvm_para.h
    index 7a94987..cf5327c 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 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 a9c2116..ec55a0b 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>
    @@ -545,6 +546,7 @@ 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)
    @@ -627,3 +629,250 @@ static __init int activate_jump_labels(void)
    return 0;
    }
    arch_initcall(activate_jump_labels);
    +
    +#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
    +
    +static struct kvm_spinlock_stats
    +{
    + u32 contention_stats[NR_CONTENTION_STATS];
    +
    +#define HISTO_BUCKETS 30
    + 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 = 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 = 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 = 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 = 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 = &__get_cpu_var(lock_waiting);
    + int cpu = smp_processor_id();
    + u64 start;
    + unsigned long flags;
    +
    + 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 a cpu by its apicid*/
    +static inline void kvm_kick_cpu(int apicid)
    +{
    + kvm_hypercall1(KVM_HC_KICK_CPU, apicid);
    +}
    +
    +/* Kick vcpu waiting on @lock->head to reach value @ticket */
    +static void kvm_unlock_kick(struct arch_spinlock *lock, __ticket_t ticket)
    +{
    + int cpu;
    + int apicid;
    +
    + 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);
    + apicid = per_cpu(x86_cpu_to_apicid, cpu);
    + kvm_kick_cpu(apicid);
    + break;
    + }
    + }
    +}
    +
    +/*
    + * Setup pv_lock_ops to exploit KVM_FEATURE_PVLOCK_KICK if present.
    + */
    +void __init kvm_spinlock_init(void)
    +{
    + if (!kvm_para_available())
    + return;
    + /* Does host kernel support KVM_FEATURE_PVLOCK_KICK? */
    + if (!kvm_para_has_feature(KVM_FEATURE_PVLOCK_KICK))
    + return;
    +
    + jump_label_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 */
    diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
    index c7b05fc..4d7a950 100644
    --- a/arch/x86/kvm/x86.c
    +++ b/arch/x86/kvm/x86.c
    @@ -5754,8 +5754,9 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)

    local_irq_disable();

    - if (vcpu->mode == EXITING_GUEST_MODE || vcpu->requests
    - || need_resched() || signal_pending(current)) {
    + if (vcpu->mode == EXITING_GUEST_MODE
    + || (vcpu->requests & ~(1UL<<KVM_REQ_PVLOCK_KICK))
    + || need_resched() || signal_pending(current)) {
    vcpu->mode = OUTSIDE_GUEST_MODE;
    smp_wmb();
    local_irq_enable();
    @@ -6711,6 +6712,7 @@ int kvm_arch_vcpu_runnable(struct kvm_vcpu *vcpu)
    !vcpu->arch.apf.halted)
    || !list_empty_careful(&vcpu->async_pf.done)
    || vcpu->arch.mp_state == KVM_MP_STATE_SIPI_RECEIVED
    + || kvm_check_request(KVM_REQ_PVLOCK_KICK, vcpu)
    || atomic_read(&vcpu->arch.nmi_queued) ||
    (kvm_arch_interrupt_allowed(vcpu) &&
    kvm_cpu_has_interrupt(vcpu));


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