linux-kernel - Re: [PATCH v2 22/43] KVM: VMX: Drop unnecessary PI logic to handle impossible conditions

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Date:   Mon, 25 Oct 2021 15:53:02 +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 22/43] KVM: VMX: Drop unnecessary PI logic to handle
 impossible conditions

On 09/10/21 04:12, Sean Christopherson wrote:
> Drop sanity checks on the validity of the previous pCPU when handling
> vCPU block/unlock for posted interrupts.  Barring a code bug or memory
> corruption, the sanity checks will never fire, and any code bug that does
> trip the WARN is all but guaranteed to completely break posted interrupts,
> i.e. should never get anywhere near production.
> 
> This is the first of several steps toward eliminating kvm_vcpu.pre_cpu.
> 
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> ---
>   arch/x86/kvm/vmx/posted_intr.c | 24 ++++++++++--------------
>   1 file changed, 10 insertions(+), 14 deletions(-)

The idea here is to avoid making things worse by not making the list 
inconsistent.  But that's impossible to do if pre_pcpu goes away, so 
fair enough.

Paolo

> diff --git a/arch/x86/kvm/vmx/posted_intr.c b/arch/x86/kvm/vmx/posted_intr.c
> index 67cbe6ab8f66..6c2110d91b06 100644
> --- a/arch/x86/kvm/vmx/posted_intr.c
> +++ b/arch/x86/kvm/vmx/posted_intr.c
> @@ -118,12 +118,10 @@ static void __pi_post_block(struct kvm_vcpu *vcpu)
>   	} while (cmpxchg64(&pi_desc->control, old.control,
>   			   new.control) != old.control);
>   
> -	if (!WARN_ON_ONCE(vcpu->pre_pcpu == -1)) {
> -		spin_lock(&per_cpu(blocked_vcpu_on_cpu_lock, vcpu->pre_pcpu));
> -		list_del(&vcpu->blocked_vcpu_list);
> -		spin_unlock(&per_cpu(blocked_vcpu_on_cpu_lock, vcpu->pre_pcpu));
> -		vcpu->pre_pcpu = -1;
> -	}
> +	spin_lock(&per_cpu(blocked_vcpu_on_cpu_lock, vcpu->pre_pcpu));
> +	list_del(&vcpu->blocked_vcpu_list);
> +	spin_unlock(&per_cpu(blocked_vcpu_on_cpu_lock, vcpu->pre_pcpu));
> +	vcpu->pre_pcpu = -1;
>   }
>   
>   /*
> @@ -153,14 +151,12 @@ int pi_pre_block(struct kvm_vcpu *vcpu)
>   
>   	WARN_ON(irqs_disabled());
>   	local_irq_disable();
> -	if (!WARN_ON_ONCE(vcpu->pre_pcpu != -1)) {
> -		vcpu->pre_pcpu = vcpu->cpu;
> -		spin_lock(&per_cpu(blocked_vcpu_on_cpu_lock, vcpu->pre_pcpu));
> -		list_add_tail(&vcpu->blocked_vcpu_list,
> -			      &per_cpu(blocked_vcpu_on_cpu,
> -				       vcpu->pre_pcpu));
> -		spin_unlock(&per_cpu(blocked_vcpu_on_cpu_lock, vcpu->pre_pcpu));
> -	}
> +
> +	vcpu->pre_pcpu = vcpu->cpu;
> +	spin_lock(&per_cpu(blocked_vcpu_on_cpu_lock, vcpu->pre_pcpu));
> +	list_add_tail(&vcpu->blocked_vcpu_list,
> +		      &per_cpu(blocked_vcpu_on_cpu, vcpu->pre_pcpu));
> +	spin_unlock(&per_cpu(blocked_vcpu_on_cpu_lock, vcpu->pre_pcpu));
>   
>   	WARN(pi_desc->sn == 1,
>   	     "Posted Interrupt Suppress Notification set before blocking");
>