Re: [PATCH v9 17/18] KVM: Update Posted-Interrupts Descriptor when vCPU is blocked

[David Matlack]

Hi Feng.

On Fri, Sep 18, 2015 at 7:29 AM, Feng Wu <feng.wu@xxxxxxxxx> wrote:
> This patch updates the Posted-Interrupts Descriptor when vCPU
> is blocked.
>
> pre-block:
> - Add the vCPU to the blocked per-CPU list
> - Set 'NV' to POSTED_INTR_WAKEUP_VECTOR
>
> post-block:
> - Remove the vCPU from the per-CPU list

I'm wondering what happens if a posted interrupt arrives at the IOMMU
after pre-block and before post-block.

In pre_block, NV is set to POSTED_INTR_WAKEUP_VECTOR. IIUC, this means
future posted interrupts will not trigger "Posted-Interrupt Processing"
(PIR will not get copied to VIRR). Instead, the IOMMU will do ON := 1,
PIR |= (1 << vector), and send POSTED_INTR_WAKEUP_VECTOR. PIWV calls
wakeup_handler which does kvm_vcpu_kick. kvm_vcpu_kick does a wait-queue
wakeup and possibly a scheduler ipi.

But the VCPU is sitting in kvm_vcpu_block. It spins and/or schedules
(wait queue) until it has a reason to wake up. I couldn't find a code
path from kvm_vcpu_block that lead to checking ON or PIR. How does the
blocked VCPU "receive" the posted interrupt? (And when does Posted-
Interrupt Processing get triggered?)

Thanks!

>
> Signed-off-by: Feng Wu <feng.wu@xxxxxxxxx>
> ---
> v9:
> - Add description for blocked_vcpu_on_cpu_lock in Documentation/virtual/kvm/locking.txt
> - Check !kvm_arch_has_assigned_device(vcpu->kvm) first, then
>   !irq_remapping_cap(IRQ_POSTING_CAP)
>
> v8:
> - Rename 'pi_pre_block' to 'pre_block'
> - Rename 'pi_post_block' to 'post_block'
> - Change some comments
> - Only add the vCPU to the blocking list when the VM has assigned devices.
>
>  Documentation/virtual/kvm/locking.txt |  12 +++
>  arch/x86/include/asm/kvm_host.h       |  13 +++
>  arch/x86/kvm/vmx.c                    | 153 ++++++++++++++++++++++++++++++++++
>  arch/x86/kvm/x86.c                    |  53 +++++++++---
>  include/linux/kvm_host.h              |   3 +
>  virt/kvm/kvm_main.c                   |   3 +
>  6 files changed, 227 insertions(+), 10 deletions(-)
>
> diff --git a/Documentation/virtual/kvm/locking.txt b/Documentation/virtual/kvm/locking.txt
> index d68af4d..19f94a6 100644
> --- a/Documentation/virtual/kvm/locking.txt
> +++ b/Documentation/virtual/kvm/locking.txt
> @@ -166,3 +166,15 @@ Comment:   The srcu read lock must be held while accessing memslots (e.g.
>                 MMIO/PIO address->device structure mapping (kvm->buses).
>                 The srcu index can be stored in kvm_vcpu->srcu_idx per vcpu
>                 if it is needed by multiple functions.
> +
> +Name:          blocked_vcpu_on_cpu_lock
> +Type:          spinlock_t
> +Arch:          x86
> +Protects:      blocked_vcpu_on_cpu
> +Comment:       This is a per-CPU lock and it is used for VT-d posted-interrupts.
> +               When VT-d posted-interrupts is supported and the VM has assigned
> +               devices, we put the blocked vCPU on the list blocked_vcpu_on_cpu
> +               protected by blocked_vcpu_on_cpu_lock, when VT-d hardware issues
> +               wakeup notification event since external interrupts from the
> +               assigned devices happens, we will find the vCPU on the list to
> +               wakeup.
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index 0ddd353..304fbb5 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -552,6 +552,8 @@ struct kvm_vcpu_arch {
>          */
>         bool write_fault_to_shadow_pgtable;
>
> +       bool halted;
> +
>         /* set at EPT violation at this point */
>         unsigned long exit_qualification;
>
> @@ -864,6 +866,17 @@ struct kvm_x86_ops {
>         /* pmu operations of sub-arch */
>         const struct kvm_pmu_ops *pmu_ops;
>
> +       /*
> +        * Architecture specific hooks for vCPU blocking due to
> +        * HLT instruction.
> +        * Returns for .pre_block():
> +        *    - 0 means continue to block the vCPU.
> +        *    - 1 means we cannot block the vCPU since some event
> +        *        happens during this period, such as, 'ON' bit in
> +        *        posted-interrupts descriptor is set.
> +        */
> +       int (*pre_block)(struct kvm_vcpu *vcpu);
> +       void (*post_block)(struct kvm_vcpu *vcpu);
>         int (*update_pi_irte)(struct kvm *kvm, unsigned int host_irq,
>                               uint32_t guest_irq, bool set);
>  };
> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
> index 902a67d..9968896 100644
> --- a/arch/x86/kvm/vmx.c
> +++ b/arch/x86/kvm/vmx.c
> @@ -879,6 +879,13 @@ 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;
> @@ -2985,6 +2992,8 @@ 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
> @@ -6121,6 +6130,25 @@ static void update_ple_window_actual_max(void)
>                                             ple_window_grow, INT_MIN);
>  }
>
> +/*
> + * Handler for POSTED_INTERRUPT_WAKEUP_VECTOR.
> + */
> +static 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));
> +}
> +
>  static __init int hardware_setup(void)
>  {
>         int r = -ENOMEM, i, msr;
> @@ -6305,6 +6333,8 @@ static __init int hardware_setup(void)
>                 kvm_x86_ops->enable_log_dirty_pt_masked = NULL;
>         }
>
> +       kvm_set_posted_intr_wakeup_handler(wakeup_handler);
> +
>         return alloc_kvm_area();
>
>  out8:
> @@ -10430,6 +10460,126 @@ static void vmx_enable_log_dirty_pt_masked(struct kvm *kvm,
>  }
>
>  /*
> + * This routine does the following things for vCPU which is going
> + * to be blocked if VT-d PI is enabled.
> + * - Store the vCPU to the wakeup list, so when interrupts happen
> + *   we can find the right vCPU to wake up.
> + * - Change the Posted-interrupt descriptor as below:
> + *      'NDST' <-- vcpu->pre_pcpu
> + *      'NV' <-- POSTED_INTR_WAKEUP_VECTOR
> + * - If 'ON' is set during this process, which means at least one
> + *   interrupt is posted for this vCPU, we cannot block it, in
> + *   this case, return 1, otherwise, return 0.
> + *
> + */
> +static int vmx_pre_block(struct kvm_vcpu *vcpu)
> +{
> +       unsigned long flags;
> +       unsigned int dest;
> +       struct pi_desc old, new;
> +       struct pi_desc *pi_desc = vcpu_to_pi_desc(vcpu);
> +
> +       if (!kvm_arch_has_assigned_device(vcpu->kvm) ||
> +               !irq_remapping_cap(IRQ_POSTING_CAP))
> +               return 0;
> +
> +       vcpu->pre_pcpu = vcpu->cpu;
> +       spin_lock_irqsave(&per_cpu(blocked_vcpu_on_cpu_lock,
> +                         vcpu->pre_pcpu), flags);
> +       list_add_tail(&vcpu->blocked_vcpu_list,
> +                     &per_cpu(blocked_vcpu_on_cpu,
> +                     vcpu->pre_pcpu));
> +       spin_unlock_irqrestore(&per_cpu(blocked_vcpu_on_cpu_lock,
> +                              vcpu->pre_pcpu), 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) {
> +                       spin_lock_irqsave(&per_cpu(blocked_vcpu_on_cpu_lock,
> +                                         vcpu->pre_pcpu), flags);
> +                       list_del(&vcpu->blocked_vcpu_list);
> +                       spin_unlock_irqrestore(
> +                                       &per_cpu(blocked_vcpu_on_cpu_lock,
> +                                       vcpu->pre_pcpu), flags);
> +                       vcpu->pre_pcpu = -1;
> +
> +                       return 1;
> +               }
> +
> +               WARN((pi_desc->sn == 1),
> +                    "Warning: SN field of posted-interrupts "
> +                    "is set before blocking\n");
> +
> +               /*
> +                * Since vCPU can be preempted during this process,
> +                * vcpu->cpu could be different with pre_pcpu, we
> +                * need to set pre_pcpu as the destination of wakeup
> +                * notification event, then we can find the right vCPU
> +                * to wakeup in wakeup handler if interrupts happen
> +                * when the vCPU is in blocked state.
> +                */
> +               dest = cpu_physical_id(vcpu->pre_pcpu);
> +
> +               if (x2apic_enabled())
> +                       new.ndst = dest;
> +               else
> +                       new.ndst = (dest << 8) & 0xFF00;
> +
> +               /* set 'NV' to 'wakeup vector' */
> +               new.nv = POSTED_INTR_WAKEUP_VECTOR;
> +       } while (cmpxchg(&pi_desc->control, old.control,
> +                       new.control) != old.control);
> +
> +       return 0;
> +}
> +
> +static void vmx_post_block(struct kvm_vcpu *vcpu)
> +{
> +       struct pi_desc *pi_desc = vcpu_to_pi_desc(vcpu);
> +       struct pi_desc old, new;
> +       unsigned int dest;
> +       unsigned long flags;
> +
> +       if (!kvm_arch_has_assigned_device(vcpu->kvm) ||
> +               !irq_remapping_cap(IRQ_POSTING_CAP))
> +               return;
> +
> +       do {
> +               old.control = new.control = pi_desc->control;
> +
> +               dest = cpu_physical_id(vcpu->cpu);
> +
> +               if (x2apic_enabled())
> +                       new.ndst = dest;
> +               else
> +                       new.ndst = (dest << 8) & 0xFF00;
> +
> +               /* Allow posting non-urgent interrupts */
> +               new.sn = 0;
> +
> +               /* set 'NV' to 'notification vector' */
> +               new.nv = POSTED_INTR_VECTOR;
> +       } while (cmpxchg(&pi_desc->control, old.control,
> +                       new.control) != old.control);
> +
> +       if(vcpu->pre_pcpu != -1) {
> +               spin_lock_irqsave(
> +                       &per_cpu(blocked_vcpu_on_cpu_lock,
> +                       vcpu->pre_pcpu), flags);
> +               list_del(&vcpu->blocked_vcpu_list);
> +               spin_unlock_irqrestore(
> +                       &per_cpu(blocked_vcpu_on_cpu_lock,
> +                       vcpu->pre_pcpu), flags);
> +               vcpu->pre_pcpu = -1;
> +       }
> +}
> +
> +/*
>   * vmx_update_pi_irte - set IRTE for Posted-Interrupts
>   *
>   * @kvm: kvm
> @@ -10620,6 +10770,9 @@ static struct kvm_x86_ops vmx_x86_ops = {
>         .flush_log_dirty = vmx_flush_log_dirty,
>         .enable_log_dirty_pt_masked = vmx_enable_log_dirty_pt_masked,
>
> +       .pre_block = vmx_pre_block,
> +       .post_block = vmx_post_block,
> +
>         .pmu_ops = &intel_pmu_ops,
>
>         .update_pi_irte = vmx_update_pi_irte,
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 58688aa..46f55b2 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -5869,7 +5869,12 @@ int kvm_vcpu_halt(struct kvm_vcpu *vcpu)
>  {
>         ++vcpu->stat.halt_exits;
>         if (irqchip_in_kernel(vcpu->kvm)) {
> -               vcpu->arch.mp_state = KVM_MP_STATE_HALTED;
> +               /* Handle posted-interrupt when vCPU is to be halted */
> +               if (!kvm_x86_ops->pre_block ||
> +                               kvm_x86_ops->pre_block(vcpu) == 0) {
> +                       vcpu->arch.halted = true;
> +                       vcpu->arch.mp_state = KVM_MP_STATE_HALTED;
> +               }
>                 return 1;
>         } else {
>                 vcpu->run->exit_reason = KVM_EXIT_HLT;
> @@ -6518,6 +6523,20 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
>                         kvm_vcpu_reload_apic_access_page(vcpu);
>         }
>
> +       /*
> +        * KVM_REQ_EVENT is not set when posted interrupts are set by
> +        * VT-d hardware, so we have to update RVI unconditionally.
> +        */
> +       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));
> +       }
> +
>         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) {
> @@ -6534,13 +6553,6 @@ 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);
>                 }
> @@ -6711,10 +6723,31 @@ static int vcpu_run(struct kvm_vcpu *vcpu)
>
>         for (;;) {
>                 if (vcpu->arch.mp_state == KVM_MP_STATE_RUNNABLE &&
> -                   !vcpu->arch.apf.halted)
> +                   !vcpu->arch.apf.halted) {
> +                       /*
> +                        * For some cases, we can get here with
> +                        * vcpu->arch.halted being true.
> +                        */
> +                       if (kvm_x86_ops->post_block && vcpu->arch.halted) {
> +                               kvm_x86_ops->post_block(vcpu);
> +                               vcpu->arch.halted = false;
> +                       }
> +
>                         r = vcpu_enter_guest(vcpu);
> -               else
> +               } else {
>                         r = vcpu_block(kvm, vcpu);
> +
> +                       /*
> +                        * post_block() must be called after
> +                        * pre_block() which is called in
> +                        * kvm_vcpu_halt().
> +                        */
> +                       if (kvm_x86_ops->post_block && vcpu->arch.halted) {
> +                               kvm_x86_ops->post_block(vcpu);
> +                               vcpu->arch.halted = false;
> +                       }
> +               }
> +
>                 if (r <= 0)
>                         break;
>
> diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> index feba1fb..bf462e7 100644
> --- a/include/linux/kvm_host.h
> +++ b/include/linux/kvm_host.h
> @@ -231,6 +231,9 @@ struct kvm_vcpu {
>         unsigned long requests;
>         unsigned long guest_debug;
>
> +       int pre_pcpu;
> +       struct list_head blocked_vcpu_list;
> +
>         struct mutex mutex;
>         struct kvm_run *run;
>
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index 8b8a444..191c7eb 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -220,6 +220,9 @@ int kvm_vcpu_init(struct kvm_vcpu *vcpu, struct kvm *kvm, unsigned id)
>         init_waitqueue_head(&vcpu->wq);
>         kvm_async_pf_vcpu_init(vcpu);
>
> +       vcpu->pre_pcpu = -1;
> +       INIT_LIST_HEAD(&vcpu->blocked_vcpu_list);
> +
>         page = alloc_page(GFP_KERNEL | __GFP_ZERO);
>         if (!page) {
>                 r = -ENOMEM;
> --
> 2.1.0
>
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