KVM: VMX: Drop pointless PI.NDST update when blocking

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).

Since commit 8b306e2f3c41 ("KVM: VMX: avoid double list add with VT-d
posted interrupts", 2017-09-27) 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 do not need pre_pcpu since the same commit.

Signed-off-by: Sean Christopherson <[email protected]>
Message-Id: <[email protected]>
Signed-off-by: Paolo Bonzini <[email protected]>

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