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    Subject[PATCH v2 28/43] KVM: VMX: Remove vCPU from PI wakeup list before updating PID.NV
    From
    Remove the vCPU from the wakeup list before updating the notification
    vector in the posted interrupt post-block helper. There is no need to
    wake the current vCPU as it is by definition not blocking. Practically
    speaking this is a nop as it only shaves a few meager cycles in the
    unlikely case that the vCPU was migrated and the previous pCPU gets a
    wakeup IRQ right before PID.NV is updated. The real motivation is to
    allow for more readable code in the future, when post-block is merged
    with vmx_vcpu_pi_load(), at which point removal from the list will be
    conditional on the old notification vector.

    Opportunistically add comments to document why KVM has a per-CPU spinlock
    that, at first glance, appears to be taken only on the owning CPU.
    Explicitly call out that the spinlock must be taken with IRQs disabled, a
    detail that was "lost" when KVM switched from spin_lock_irqsave() to
    spin_lock(), with IRQs disabled for the entirety of the relevant path.

    Signed-off-by: Sean Christopherson <seanjc@google.com>
    ---
    arch/x86/kvm/vmx/posted_intr.c | 49 +++++++++++++++++++++++-----------
    1 file changed, 33 insertions(+), 16 deletions(-)

    diff --git a/arch/x86/kvm/vmx/posted_intr.c b/arch/x86/kvm/vmx/posted_intr.c
    index 2b2206339174..901b7a5f7777 100644
    --- a/arch/x86/kvm/vmx/posted_intr.c
    +++ b/arch/x86/kvm/vmx/posted_intr.c
    @@ -10,10 +10,22 @@
    #include "vmx.h"

    /*
    - * We maintain a per-CPU linked-list of vCPU, so in wakeup_handler() we
    - * can find which vCPU should be waken up.
    + * Maintain a per-CPU list of vCPUs that need to be awakened by wakeup_handler()
    + * when a WAKEUP_VECTOR interrupted is posted. vCPUs are added to the list when
    + * the vCPU is scheduled out and is blocking (e.g. in HLT) with IRQs enabled.
    + * The vCPUs posted interrupt descriptor is updated at the same time to set its
    + * notification vector to WAKEUP_VECTOR, so that posted interrupt from devices
    + * wake the target vCPUs. vCPUs are removed from the list and the notification
    + * vector is reset when the vCPU is scheduled in.
    */
    static DEFINE_PER_CPU(struct list_head, blocked_vcpu_on_cpu);
    +/*
    + * Protect the per-CPU list with a per-CPU spinlock to handle task migration.
    + * When a blocking vCPU is awakened _and_ migrated to a different pCPU, the
    + * ->sched_in() path will need to take the vCPU off the list of the _previous_
    + * CPU. IRQs must be disabled when taking this lock, otherwise deadlock will
    + * occur if a wakeup IRQ arrives and attempts to acquire the lock.
    + */
    static DEFINE_PER_CPU(spinlock_t, blocked_vcpu_on_cpu_lock);

    static inline struct pi_desc *vcpu_to_pi_desc(struct kvm_vcpu *vcpu)
    @@ -101,23 +113,28 @@ static void __pi_post_block(struct kvm_vcpu *vcpu)
    WARN(pi_desc->nv != POSTED_INTR_WAKEUP_VECTOR,
    "Wakeup handler not enabled while the vCPU was blocking");

    - dest = cpu_physical_id(vcpu->cpu);
    - if (!x2apic_mode)
    - dest = (dest << 8) & 0xFF00;
    -
    - do {
    - old.control = new.control = READ_ONCE(pi_desc->control);
    -
    - new.ndst = dest;
    -
    - /* set 'NV' to 'notification vector' */
    - new.nv = POSTED_INTR_VECTOR;
    - } while (cmpxchg64(&pi_desc->control, old.control,
    - new.control) != old.control);
    -
    + /*
    + * Remove the vCPU from the wakeup list of the _previous_ pCPU, which
    + * will not be the same as the current pCPU if the task was migrated.
    + */
    spin_lock(&per_cpu(blocked_vcpu_on_cpu_lock, vcpu->pre_pcpu));
    list_del(&vcpu->blocked_vcpu_list);
    spin_unlock(&per_cpu(blocked_vcpu_on_cpu_lock, vcpu->pre_pcpu));
    +
    + dest = cpu_physical_id(vcpu->cpu);
    + if (!x2apic_mode)
    + dest = (dest << 8) & 0xFF00;
    +
    + do {
    + old.control = new.control = READ_ONCE(pi_desc->control);
    +
    + new.ndst = dest;
    +
    + /* set 'NV' to 'notification vector' */
    + new.nv = POSTED_INTR_VECTOR;
    + } while (cmpxchg64(&pi_desc->control, old.control,
    + new.control) != old.control);
    +
    vcpu->pre_pcpu = -1;
    }

    --
    2.33.0.882.g93a45727a2-goog
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