linux-kernel - Re: [PATCH] KVM: x86: don't notify userspace IOAPIC on edge-triggered interrupt EOI

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Date:   Tue, 18 Feb 2020 01:41:33 +0000
From:   linmiaohe <linmiaohe@...wei.com>
To:     Vitaly Kuznetsov <vkuznets@...hat.com>
CC:     "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "x86@...nel.org" <x86@...nel.org>,
        "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "rkrcmar@...hat.com" <rkrcmar@...hat.com>,
        "sean.j.christopherson@...el.com" <sean.j.christopherson@...el.com>,
        "wanpengli@...cent.com" <wanpengli@...cent.com>,
        "jmattson@...gle.com" <jmattson@...gle.com>,
        "joro@...tes.org" <joro@...tes.org>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "bp@...en8.de" <bp@...en8.de>, "hpa@...or.com" <hpa@...or.com>
Subject: Re: [PATCH] KVM: x86: don't notify userspace IOAPIC on edge-triggered
 interrupt EOI

Vitaly Kuznetsov <vkuznets@...hat.com> writes:
>linmiaohe <linmiaohe@...wei.com> writes:
>
>> @@ -417,7 +417,7 @@ void kvm_scan_ioapic_routes(struct kvm_vcpu *vcpu,
>>  
>>  			kvm_set_msi_irq(vcpu->kvm, entry, &irq);
>>  
>> -			if (irq.level &&
>> +			if (irq.trig_mode &&
>>  			    kvm_apic_match_dest(vcpu, NULL, APIC_DEST_NOSHORT,
>>  						irq.dest_id, irq.dest_mode))
>>  				__set_bit(irq.vector, ioapic_handled_vectors);
>
>Assuming Radim's comment (13db77347db1) is correct, the change in
>3159d36ad799 looks wrong and your patch restores the status quo. Actually, kvm_set_msi_irq() always sets irq->level = 1 so checking it is pointless.
>
>Reviewed-by: Vitaly Kuznetsov <vkuznets@...hat.com>

Thanks for review.

>
> (but it is actually possible that there's a buggy userspace out there which expects EOI notifications; we won't find out unless we try to fix the bug).
>

Yeh, there may be a buggy userspace hidden from this unexpected EOI notifications. It may not be worth enough to fix it as we may spend many time
to catch the bug.
Perhaps we should only remove the pointless checking of irq->level for cleanup. :)