Re: [v3 24/26] KVM: Update Posted-Interrupts Descriptor when vCPU is blocked

[Marcelo Tosatti <mtosatti@redhat.com>]

On Fri, Dec 12, 2014 at 11:14:58PM +0800, Feng Wu wrote:
> This patch updates the Posted-Interrupts Descriptor when vCPU
> is blocked.
> 
> pre-block:
> - Add the vCPU to the blocked per-CPU list
> - Clear 'SN'
> - Set 'NV' to POSTED_INTR_WAKEUP_VECTOR
> 
> post-block:
> - Remove the vCPU from the per-CPU list
> 
> Signed-off-by: Feng Wu <feng.wu@intel.com>
> ---
>  arch/x86/include/asm/kvm_host.h |  2 +
>  arch/x86/kvm/vmx.c              | 96 +++++++++++++++++++++++++++++++++++++++++
>  arch/x86/kvm/x86.c              | 22 +++++++---
>  include/linux/kvm_host.h        |  4 ++
>  virt/kvm/kvm_main.c             |  6 +++
>  5 files changed, 123 insertions(+), 7 deletions(-)
> 
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index 13e3e40..32c110a 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -101,6 +101,8 @@ static inline gfn_t gfn_to_index(gfn_t gfn, gfn_t base_gfn, int level)
>  
>  #define ASYNC_PF_PER_VCPU 64
>  
> +extern void (*wakeup_handler_callback)(void);
> +
>  enum kvm_reg {
>  	VCPU_REGS_RAX = 0,
>  	VCPU_REGS_RCX = 1,
> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
> index bf2e6cd..a1c83a2 100644
> --- a/arch/x86/kvm/vmx.c
> +++ b/arch/x86/kvm/vmx.c
> @@ -832,6 +832,13 @@ static DEFINE_PER_CPU(struct vmcs *, current_vmcs);
>  static DEFINE_PER_CPU(struct list_head, loaded_vmcss_on_cpu);
>  static DEFINE_PER_CPU(struct desc_ptr, host_gdt);
>  
> +/*
> + * We maintian a per-CPU linked-list of vCPU, so in wakeup_handler() we
> + * can find which vCPU should be waken up.
> + */
> +static DEFINE_PER_CPU(struct list_head, blocked_vcpu_on_cpu);
> +static DEFINE_PER_CPU(spinlock_t, blocked_vcpu_on_cpu_lock);
> +
>  static unsigned long *vmx_io_bitmap_a;
>  static unsigned long *vmx_io_bitmap_b;
>  static unsigned long *vmx_msr_bitmap_legacy;
> @@ -1921,6 +1928,7 @@ static void vmx_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
>  		struct pi_desc *pi_desc = vcpu_to_pi_desc(vcpu);
>  		struct pi_desc old, new;
>  		unsigned int dest;
> +		unsigned long flags;
>  
>  		memset(&old, 0, sizeof(old));
>  		memset(&new, 0, sizeof(new));
> @@ -1942,6 +1950,20 @@ static void vmx_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
>  			new.nv = POSTED_INTR_VECTOR;
>  		} while (cmpxchg(&pi_desc->control, old.control,
>  				new.control) != old.control);
> +
> +		/*
> +		 * Delete the vCPU from the related wakeup queue
> +		 * if we are resuming from blocked state
> +		 */
> +		if (vcpu->blocked) {
> +			vcpu->blocked = false;
> +			spin_lock_irqsave(&per_cpu(blocked_vcpu_on_cpu_lock,
> +				vcpu->wakeup_cpu), flags);
> +			list_del(&vcpu->blocked_vcpu_list);
> +			spin_unlock_irqrestore(&per_cpu(blocked_vcpu_on_cpu_lock,
> +				vcpu->wakeup_cpu), flags);
> +			vcpu->wakeup_cpu = -1;
> +		}
>  	}
>  }
>  
> @@ -1950,6 +1972,9 @@ static void vmx_vcpu_put(struct kvm_vcpu *vcpu)
>  	if (irq_remapping_cap(IRQ_POSTING_CAP)) {
>  		struct pi_desc *pi_desc = vcpu_to_pi_desc(vcpu);
>  		struct pi_desc old, new;
> +		unsigned long flags;
> +		int cpu;
> +		struct cpumask cpu_others_mask;
>  
>  		memset(&old, 0, sizeof(old));
>  		memset(&new, 0, sizeof(new));
> @@ -1961,6 +1986,54 @@ static void vmx_vcpu_put(struct kvm_vcpu *vcpu)
>  				pi_set_sn(&new);
>  			} while (cmpxchg(&pi_desc->control, old.control,
>  					new.control) != old.control);
> +		} else if (vcpu->blocked) {
> +			/*
> +			 * The vcpu is blocked on the wait queue.
> +			 * Store the blocked vCPU on the list of the
> +			 * vcpu->wakeup_cpu, which is the destination
> +			 * of the wake-up notification event.
> +			 */
> +			vcpu->wakeup_cpu = vcpu->cpu;
> +			spin_lock_irqsave(&per_cpu(blocked_vcpu_on_cpu_lock,
> +					  vcpu->wakeup_cpu), flags);
> +			list_add_tail(&vcpu->blocked_vcpu_list,
> +				      &per_cpu(blocked_vcpu_on_cpu,
> +				      vcpu->wakeup_cpu));
> +			spin_unlock_irqrestore(
> +					&per_cpu(blocked_vcpu_on_cpu_lock,
> +					vcpu->wakeup_cpu), flags);
> +
> +			do {
> +				old.control = new.control = pi_desc->control;
> +
> +				/*
> +				 * We should not block the vCPU if
> +				 * an interrupt is posted for it.
> +				 */
> +				if (pi_test_on(pi_desc) == 1) {
> +					/*
> +					 * We need schedule the wakeup worker
> +					 * on a different cpu other than
> +					 * vcpu->cpu, because in some case,
> +					 * schedule_work() will call
> +					 * try_to_wake_up() which needs acquire
> +					 * the rq lock. This can cause deadlock.
> +					 */
> +					cpumask_copy(&cpu_others_mask,
> +						     cpu_online_mask);
> +					cpu_clear(vcpu->cpu, cpu_others_mask);
> +					cpu = any_online_cpu(cpu_others_mask);
> +
> +					schedule_work_on(cpu,
> +							 &vcpu->wakeup_worker);
> +				}
> +
> +				pi_clear_sn(&new);
> +
> +				/* set 'NV' to 'wakeup vector' */
> +				new.nv = POSTED_INTR_WAKEUP_VECTOR;
> +			} while (cmpxchg(&pi_desc->control, old.control,
> +				new.control) != old.control);
>  		}

This can be done exclusively on HLT emulation, correct? (that is, on
entry to HLT and exit from HLT).

If the vcpu is scheduled out for any other reason (transition to
userspace or transition to other thread), it will eventually resume
execution. And in that case, continuation of execution does not depend
on the event (VT-d interrupt) notification.

There is a race window with the code above, I believe.

>  	}
>  
> @@ -2842,6 +2915,8 @@ static int hardware_enable(void)
>  		return -EBUSY;
>  
>  	INIT_LIST_HEAD(&per_cpu(loaded_vmcss_on_cpu, cpu));
> +	INIT_LIST_HEAD(&per_cpu(blocked_vcpu_on_cpu, cpu));
> +	spin_lock_init(&per_cpu(blocked_vcpu_on_cpu_lock, cpu));
>  
>  	/*
>  	 * Now we can enable the vmclear operation in kdump
> @@ -9315,6 +9390,25 @@ static struct kvm_x86_ops vmx_x86_ops = {
>  	.pi_set_sn = vmx_pi_set_sn,
>  };
>  
> +/*
> + * Handler for POSTED_INTERRUPT_WAKEUP_VECTOR.
> + */
> +void wakeup_handler(void)
> +{
> +	struct kvm_vcpu *vcpu;
> +	int cpu = smp_processor_id();
> +
> +	spin_lock(&per_cpu(blocked_vcpu_on_cpu_lock, cpu));
> +	list_for_each_entry(vcpu, &per_cpu(blocked_vcpu_on_cpu, cpu),
> +			blocked_vcpu_list) {
> +		struct pi_desc *pi_desc = vcpu_to_pi_desc(vcpu);
> +
> +		if (pi_test_on(pi_desc) == 1)
> +			kvm_vcpu_kick(vcpu);
> +	}
> +	spin_unlock(&per_cpu(blocked_vcpu_on_cpu_lock, cpu));
> +}

Looping through all blocked vcpus does not scale:
Can you allocate more vectors and then multiplex those
vectors amongst the HLT'ed vcpus? 

It seems there is a bunch free:

commit 52aec3308db85f4e9f5c8b9f5dc4fbd0138c6fa4
Author: Alex Shi <alex.shi@intel.com>
Date:   Thu Jun 28 09:02:23 2012 +0800

    x86/tlb: replace INVALIDATE_TLB_VECTOR by CALL_FUNCTION_VECTOR

Can you add only vcpus which have posted IRTEs that point to this pCPU
to the HLT'ed vcpu lists? (so for example, vcpus without assigned
devices are not part of the list).

> +
>  static int __init vmx_init(void)
>  {
>  	int r, i, msr;
> @@ -9429,6 +9523,8 @@ static int __init vmx_init(void)
>  
>  	update_ple_window_actual_max();
>  
> +	wakeup_handler_callback = wakeup_handler;
> +
>  	return 0;
>  
>  out7:
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 0033df3..1551a46 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -6152,6 +6152,21 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
>  			kvm_vcpu_reload_apic_access_page(vcpu);
>  	}
>  
> +	/*
> +	 * Since posted-interrupts can be set by VT-d HW now, in this
> +	 * case, KVM_REQ_EVENT is not set. We move the following
> +	 * operations out of the if statement.
> +	 */
> +	if (kvm_lapic_enabled(vcpu)) {
> +		/*
> +		 * Update architecture specific hints for APIC
> +		 * virtual interrupt delivery.
> +		 */
> +		if (kvm_x86_ops->hwapic_irr_update)
> +			kvm_x86_ops->hwapic_irr_update(vcpu,
> +				kvm_lapic_find_highest_irr(vcpu));
> +	}
> +

This is a hot fast path. You can set KVM_REQ_EVENT from wakeup_handler.

>  	if (kvm_check_request(KVM_REQ_EVENT, vcpu) || req_int_win) {
>  		kvm_apic_accept_events(vcpu);
>  		if (vcpu->arch.mp_state == KVM_MP_STATE_INIT_RECEIVED) {
> @@ -6168,13 +6183,6 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
>  			kvm_x86_ops->enable_irq_window(vcpu);
>  
>  		if (kvm_lapic_enabled(vcpu)) {
> -			/*
> -			 * Update architecture specific hints for APIC
> -			 * virtual interrupt delivery.
> -			 */
> -			if (kvm_x86_ops->hwapic_irr_update)
> -				kvm_x86_ops->hwapic_irr_update(vcpu,
> -					kvm_lapic_find_highest_irr(vcpu));
>  			update_cr8_intercept(vcpu);
>  			kvm_lapic_sync_to_vapic(vcpu);
>  		}
> diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> index 3d7242c..d981d16 100644
> --- a/include/linux/kvm_host.h
> +++ b/include/linux/kvm_host.h
> @@ -239,6 +239,9 @@ struct kvm_vcpu {
>  	unsigned long requests;
>  	unsigned long guest_debug;
>  
> +	int wakeup_cpu;
> +	struct list_head blocked_vcpu_list;
> +
>  	struct mutex mutex;
>  	struct kvm_run *run;
>  
> @@ -282,6 +285,7 @@ struct kvm_vcpu {
>  	} spin_loop;
>  #endif
>  	bool preempted;
> +	bool blocked;
>  	struct kvm_vcpu_arch arch;
>  };

Please remove blocked and wakeup_cpu, they should not be necessary.

> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index ba53fd6..6deb994 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -233,6 +233,9 @@ int kvm_vcpu_init(struct kvm_vcpu *vcpu, struct kvm *kvm, unsigned id)
>  
>  	INIT_WORK(&vcpu->wakeup_worker, wakeup_thread);
>  
> +	vcpu->wakeup_cpu = -1;
> +	INIT_LIST_HEAD(&vcpu->blocked_vcpu_list);
> +
>  	page = alloc_page(GFP_KERNEL | __GFP_ZERO);
>  	if (!page) {
>  		r = -ENOMEM;
> @@ -243,6 +246,7 @@ int kvm_vcpu_init(struct kvm_vcpu *vcpu, struct kvm *kvm, unsigned id)
>  	kvm_vcpu_set_in_spin_loop(vcpu, false);
>  	kvm_vcpu_set_dy_eligible(vcpu, false);
>  	vcpu->preempted = false;
> +	vcpu->blocked = false;
>  
>  	r = kvm_arch_vcpu_init(vcpu);
>  	if (r < 0)
> @@ -1752,6 +1756,7 @@ void kvm_vcpu_block(struct kvm_vcpu *vcpu)
>  	DEFINE_WAIT(wait);
>  
>  	for (;;) {
> +		vcpu->blocked = true;
>  		prepare_to_wait(&vcpu->wq, &wait, TASK_INTERRUPTIBLE);
>  
>  		if (kvm_arch_vcpu_runnable(vcpu)) {
> @@ -1767,6 +1772,7 @@ void kvm_vcpu_block(struct kvm_vcpu *vcpu)
>  	}
>  
>  	finish_wait(&vcpu->wq, &wait);
> +	vcpu->blocked = false;
>  }
>  EXPORT_SYMBOL_GPL(kvm_vcpu_block);
>  
> -- 
> 1.9.1
> 
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