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

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 <[email protected]>
>     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.
>