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

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Message-ID: <YXlhnkp4/XJCCO0R@google.com>
Date:   Wed, 27 Oct 2021 14:26:38 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     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>,
        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 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@...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.
>