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

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Message-ID: <18e6a656-f583-0ad4-6770-9678be3f5cf4@redhat.com>
Date:   Mon, 25 Oct 2021 16:01:42 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>,
        Marc Zyngier <maz@...nel.org>,
        Huacai Chen <chenhuacai@...nel.org>,
        Aleksandar Markovic <aleksandar.qemu.devel@...il.com>,
        Paul Mackerras <paulus@...abs.org>,
        Anup Patel <anup.patel@....com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        Janosch Frank <frankja@...ux.ibm.com>
Cc:     James Morse <james.morse@....com>,
        Alexandru Elisei <alexandru.elisei@....com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Atish Patra <atish.patra@....com>,
        David Hildenbrand <david@...hat.com>,
        Cornelia Huck <cohuck@...hat.com>,
        Claudio Imbrenda <imbrenda@...ux.ibm.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.cs.columbia.edu,
        linux-mips@...r.kernel.org, kvm@...r.kernel.org,
        kvm-ppc@...r.kernel.org, kvm-riscv@...ts.infradead.org,
        linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org,
        David Matlack <dmatlack@...gle.com>,
        Oliver Upton <oupton@...gle.com>,
        Jing Zhang <jingzhangos@...gle.com>
Subject: Re: [PATCH v2 24/43] KVM: VMX: Drop pointless PI.NDST update when
 blocking

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@...hat.com>
     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.