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.
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