On Mon, Oct 25, 2021, Paolo Bonzini wrote: > 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: Ya, I found that commit when digging around for different commit in the series and forgot to come back to this changelog. I'll incorporate this info into the next version. > 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. >
