linux-kernel - Re: [PATCH] KVM: x86: Update irr_pending when setting APIC state with APICv disabled

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Message-ID: <ZytLLD6wbQgNIHuL@intel.com>
Date: Wed, 6 Nov 2024 18:55:40 +0800
From: Chao Gao <chao.gao@...el.com>
To: Sean Christopherson <seanjc@...gle.com>
CC: Paolo Bonzini <pbonzini@...hat.com>, <kvm@...r.kernel.org>,
	<linux-kernel@...r.kernel.org>, Yong He <zhuangel570@...il.com>, "Maxim
 Levitsky" <mlevitsk@...hat.com>
Subject: Re: [PATCH] KVM: x86: Update irr_pending when setting APIC state
 with APICv disabled

>Furthermore, in addition to introducing this issue, commit 755c2bf87860 also
>papered over the underlying bug: KVM doesn't ensure CPUs and devices see APICv
>as disabled prior to searching the IRR.  Waiting until KVM emulates EOI to update
>irr_pending works because KVM won't emulate EOI until after refresh_apicv_exec_ctrl(),
>and because there are plenty of memory barries in between, but leaving irr_pending
>set is basically hacking around bad ordering, which I _think_ can be fixed by:
>
>diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
>index 83fe0a78146f..85d330b56c7e 100644
>--- a/arch/x86/kvm/x86.c
>+++ b/arch/x86/kvm/x86.c
>@@ -10548,8 +10548,8 @@ void __kvm_vcpu_update_apicv(struct kvm_vcpu *vcpu)
>                goto out;
> 
>        apic->apicv_active = activate;
>-       kvm_apic_update_apicv(vcpu);
>        kvm_x86_call(refresh_apicv_exec_ctrl)(vcpu);
>+       kvm_apic_update_apicv(vcpu);

I may miss something important. how does this change ensure CPUs and devices see
APICv as disabled (thus won't manipulate the vCPU's IRR)? Other CPUs when
performing IPI virtualization just looks up the PID_table while IOMMU looks up
the IRTE table. ->refresh_apicv_exec_ctrl() doesn't change any of them.