linux-kernel - Re: [PATCH] svm: Fix improper check when deactivate AVIC

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Message-ID: <8663e279-5ea2-a3a9-9ecc-7b7226ab48bc@redhat.com>
Date:   Wed, 30 Jan 2019 17:22:43 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     "Suthikulpanit, Suravee" <Suravee.Suthikulpanit@....com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "x86@...nel.org" <x86@...nel.org>
Cc:     "joro@...tes.org" <joro@...tes.org>,
        "rkrcmar@...hat.com" <rkrcmar@...hat.com>,
        "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] svm: Fix improper check when deactivate AVIC

On 29/01/19 09:09, Suthikulpanit, Suravee wrote:
> The function svm_refresh_apicv_exec_ctrl() always returning prematurely
> as kvm_vcpu_apicv_active() always return false when calling from
> the function arch/x86/kvm/x86.c:kvm_vcpu_deactivate_apicv().
> This is because the apicv_active is set to false just before calling
> refresh_apicv_exec_ctrl().
> 
> Also, we need to mark VMCB_AVIC bit as dirty instead of VMCB_INTR.
> 
> So, fix svm_refresh_apicv_exec_ctrl() to properly deactivate AVIC.
> 
> Fixes: 67034bb9dd5e ('KVM: SVM: Add irqchip_split() checks before enabling AVIC')

Having the lapic_in_kernel check in your code is a bit ugly - the caller
should check it.  And actually, even though it doesn't check it right
now, svm_refresh_apicv_exec_ctrl is only reachable if lapic_in_kernel
returns true; see the KVM_CAP_HYPERV_SYNIC case of
kvm_vcpu_ioctl_enable_cap.

What about this simpler patch:

diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
index 01b66bbcdd7a..74ceda470eae 100644
--- a/arch/x86/kvm/svm.c
+++ b/arch/x86/kvm/svm.c
@@ -5122,11 +5122,11 @@ static void svm_refresh_apicv_exec_ctrl(
 	struct vcpu_svm *svm = to_svm(vcpu);
 	struct vmcb *vmcb = svm->vmcb;

-	if (!kvm_vcpu_apicv_active(&svm->vcpu))
-		return;
-
-	vmcb->control.int_ctl &= ~AVIC_ENABLE_MASK;
-	mark_dirty(vmcb, VMCB_INTR);
+	if (kvm_vcpu_apicv_active(vcpu))
+		vmcb->control.int_ctl |= AVIC_ENABLE_MASK;
+	else
+		vmcb->control.int_ctl &= ~AVIC_ENABLE_MASK;
+	mark_dirty(vmcb, VMCB_AVIC);
 }

 static void svm_load_eoi_exitmap(struct kvm_vcpu *vcpu, u64
*eoi_exit_bitmap)

?

Thanks,

Paolo