linux-kernel - Re: [PATCH v2 21/43] KVM: VMX: Clean up PI pre/post-block WARNs

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Date:   Thu, 28 Oct 2021 13:20:20 +0300
From:   Maxim Levitsky <mlevitsk@...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>,
        Paolo Bonzini <pbonzini@...hat.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 21/43] KVM: VMX: Clean up PI pre/post-block WARNs

On Fri, 2021-10-08 at 19:12 -0700, Sean Christopherson wrote:
> Move the WARN sanity checks out of the PI descriptor update loop so as
> not to spam the kernel log if the condition is violated and the update
> takes multiple attempts due to another writer.  This also eliminates a
> few extra uops from the retry path.
> 
> Technically not checking every attempt could mean KVM will now fail to
> WARN in a scenario that would have failed before, but any such failure
> would be inherently racy as some other agent (CPU or device) would have
> to concurrent modify the PI descriptor.
> 
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> ---
>  arch/x86/kvm/vmx/posted_intr.c | 12 ++++++------
>  1 file changed, 6 insertions(+), 6 deletions(-)
> 
> diff --git a/arch/x86/kvm/vmx/posted_intr.c b/arch/x86/kvm/vmx/posted_intr.c
> index 351666c41bbc..67cbe6ab8f66 100644
> --- a/arch/x86/kvm/vmx/posted_intr.c
> +++ b/arch/x86/kvm/vmx/posted_intr.c
> @@ -100,10 +100,11 @@ static void __pi_post_block(struct kvm_vcpu *vcpu)
>  	struct pi_desc old, new;
>  	unsigned int dest;
>  
> +	WARN(pi_desc->nv != POSTED_INTR_WAKEUP_VECTOR,
> +	     "Wakeup handler not enabled while the vCPU was blocking");
> +
>  	do {
>  		old.control = new.control = pi_desc->control;
> -		WARN(old.nv != POSTED_INTR_WAKEUP_VECTOR,
> -		     "Wakeup handler not enabled while the VCPU is blocked\n");
>  
>  		dest = cpu_physical_id(vcpu->cpu);
>  
> @@ -161,13 +162,12 @@ int pi_pre_block(struct kvm_vcpu *vcpu)
>  		spin_unlock(&per_cpu(blocked_vcpu_on_cpu_lock, vcpu->pre_pcpu));
>  	}
>  
> +	WARN(pi_desc->sn == 1,
> +	     "Posted Interrupt Suppress Notification set before blocking");
> +
>  	do {
>  		old.control = new.control = pi_desc->control;
>  
> -		WARN((pi_desc->sn == 1),
> -		     "Warning: SN field of posted-interrupts "
> -		     "is set before blocking\n");
> -
>  		/*
>  		 * Since vCPU can be preempted during this process,
>  		 * vcpu->cpu could be different with pre_pcpu, we

Don't know for sure if this is desired. I'll would just use WARN_ON_ONCE instead
if the warning spams the log.

If there is a race I would rather want to catch it even if rare.

Best regards,
	Maxim Levitsky