Re: [PATCH v2 24/43] KVM: VMX: Drop pointless PI.NDST update when blocking

[Paolo Bonzini]

On 09/10/21 04:12, Sean Christopherson wrote:
> Don't update Posted Interrupt's NDST, a.k.a. the target pCPU, in the
> pre-block path, as NDST is guaranteed to be up-to-date.  The comment
> about the vCPU being preempted during the update is simply wrong, as the
> update path runs with IRQs disabled (from before snapshotting vcpu->cpu,
> until after the update completes).

Right, it didn't as of commit bf9f6ac8d74969690df1485b33b7c238ca9f2269 
(when VT-d posted interrupts were introduced).

The interrupt disable/enable pair was added in the same commit that 
motivated the introduction of the sanity checks:

    commit 8b306e2f3c41939ea528e6174c88cfbfff893ce1
    Author: Paolo Bonzini <pbonzini@xxxxxxxxxx>
    Date:   Tue Jun 6 12:57:05 2017 +0200

    KVM: VMX: avoid double list add with VT-d posted interrupts

    In some cases, for example involving hot-unplug of assigned
    devices, pi_post_block can forget to remove the vCPU from the
    blocked_vcpu_list.  When this happens, the next call to
    pi_pre_block corrupts the list.

    Fix this in two ways.  First, check vcpu->pre_pcpu in pi_pre_block
    and WARN instead of adding the element twice in the list.  Second,
    always do the list removal in pi_post_block if vcpu->pre_pcpu is
    set (not -1).

    The new code keeps interrupts disabled for the whole duration of
    pi_pre_block/pi_post_block.  This is not strictly necessary, but
    easier to follow.  For the same reason, PI.ON is checked only
    after the cmpxchg, and to handle it we just call the post-block
    code.  This removes duplication of the list removal code.

At the time, I didn't notice the now useless NDST update.

Paolo

> The vCPU can get preempted_before_  the update starts, but not during.
> And if the vCPU is preempted before, vmx_vcpu_pi_load() is responsible
> for updating NDST when the vCPU is scheduled back in.  In that case, the
> check against the wakeup vector in vmx_vcpu_pi_load() cannot be true as
> that would require the notification vector to have been set to the wakeup
> vector_before_  blocking.
>
> Opportunistically switch to using vcpu->cpu for the list/lock lookups,
> which presumably used pre_pcpu only for some phantom preemption logic.