linux-kernel - Re: [PATCH v2 3/5] KVM: SVM: fix race between interrupt delivery and AVIC inhibition

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Message-ID: <6a11edec-c29a-95df-393e-363e1af46257@redhat.com>
Date:   Fri, 7 Jan 2022 16:32:44 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Maxim Levitsky <mlevitsk@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>
Cc:     kvm@...r.kernel.org, Jim Mattson <jmattson@...gle.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Joerg Roedel <joro@...tes.org>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Borislav Petkov <bp@...en8.de>, linux-kernel@...r.kernel.org,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Ingo Molnar <mingo@...hat.com>
Subject: Re: [PATCH v2 3/5] KVM: SVM: fix race between interrupt delivery and
 AVIC inhibition

On 1/5/22 12:03, Maxim Levitsky wrote:
>>> -	if (!vcpu->arch.apicv_active)
>>> -		return -1;
>>> -
>>> +	/*
>>> +	 * Below, we have to handle anyway the case of AVIC being disabled
>>> +	 * in the middle of this function, and there is hardly any overhead
>>> +	 * if AVIC is disabled.  So, we do not bother returning -1 and handle
>>> +	 * the kick ourselves for disabled APICv.
>> Hmm, my preference would be to keep the "return -1" even though apicv_active must
>> be rechecked.  That would help highlight that returning "failure" after this point
>> is not an option as it would result in kvm_lapic_set_irr() being called twice.
> I don't mind either - this will fix the tracepoint I recently added to report the
> number of interrupts that were delivered by AVIC/APICv - with this patch,
> all of them count as such.

The reasoning here is that, unlike VMX, we have to react anyway to 
vcpu->arch.apicv_active becoming false halfway through the function.

Removing the early return means that there's one less case of load 
(mis)reordering that the reader has to check.  So I really would prefer 
to remove it.

Agreed with the other feedback.

Paolo