linux-kernel - [PATCH v6 13/17] KVM: SVM: Introduce hybrid-AVIC mode

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Date:   Thu, 19 May 2022 05:27:05 -0500
From:   Suravee Suthikulpanit <suravee.suthikulpanit@....com>
To:     <linux-kernel@...r.kernel.org>, <kvm@...r.kernel.org>
CC:     <pbonzini@...hat.com>, <mlevitsk@...hat.com>, <seanjc@...gle.com>,
        <joro@...tes.org>, <jon.grimm@....com>, <wei.huang2@....com>,
        <terry.bowman@....com>,
        Suravee Suthikulpanit <suravee.suthikulpanit@....com>
Subject: [PATCH v6 13/17] KVM: SVM: Introduce hybrid-AVIC mode

Currently, AVIC is inhibited when booting a VM w/ x2APIC support.
because AVIC cannot virtualize x2APIC MSR register accesses.
However, the AVIC doorbell can be used to accelerate interrupt
injection into a running vCPU, while all guest accesses to x2APIC MSRs
will be intercepted and emulated by KVM.

With hybrid-AVIC support, the APICV_INHIBIT_REASON_X2APIC is
no longer enforced.

Suggested-by: Maxim Levitsky <mlevitsk@...hat.com>
Reviewed-by: Maxim Levitsky <mlevitsk@...hat.com>
Signed-off-by: Suravee Suthikulpanit <suravee.suthikulpanit@....com>
---
 arch/x86/include/asm/kvm_host.h |  1 -
 arch/x86/kvm/svm/avic.c         | 13 +++++++++++--
 arch/x86/kvm/svm/svm.c          |  9 ---------
 3 files changed, 11 insertions(+), 12 deletions(-)

diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index c59fea4bdb6e..da03111b05f6 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -1051,7 +1051,6 @@ enum kvm_apicv_inhibit {
 	APICV_INHIBIT_REASON_NESTED,
 	APICV_INHIBIT_REASON_IRQWIN,
 	APICV_INHIBIT_REASON_PIT_REINJ,
-	APICV_INHIBIT_REASON_X2APIC,
 	APICV_INHIBIT_REASON_BLOCKIRQ,
 	APICV_INHIBIT_REASON_ABSENT,
 	APICV_INHIBIT_REASON_SEV,
diff --git a/arch/x86/kvm/svm/avic.c b/arch/x86/kvm/svm/avic.c
index 2d9455338b1f..bac876bb1cf1 100644
--- a/arch/x86/kvm/svm/avic.c
+++ b/arch/x86/kvm/svm/avic.c
@@ -71,12 +71,22 @@ static void avic_activate_vmcb(struct vcpu_svm *svm)
 	vmcb->control.avic_physical_id &= ~AVIC_PHYSICAL_MAX_INDEX_MASK;
 
 	vmcb->control.int_ctl |= AVIC_ENABLE_MASK;
-	if (apic_x2apic_mode(svm->vcpu.arch.apic)) {
+
+	/* Note:
+	 * KVM can support hybrid-AVIC mode, where KVM emulates x2APIC
+	 * MSR accesses, while interrupt injection to a running vCPU
+	 * can be achieved using AVIC doorbell. The AVIC hardware still
+	 * accelerate MMIO accesses, but this does not cause any harm
+	 * as the guest is not supposed to access xAPIC mmio when uses x2APIC.
+	 */
+	if (apic_x2apic_mode(svm->vcpu.arch.apic) &&
+	    (avic_mode == AVIC_MODE_X2)) {
 		vmcb->control.int_ctl |= X2APIC_MODE_MASK;
 		vmcb->control.avic_physical_id |= X2AVIC_MAX_PHYSICAL_ID;
 		/* Disabling MSR intercept for x2APIC registers */
 		svm_set_x2apic_msr_interception(svm, false);
 	} else {
+		/* For xAVIC and hybrid-xAVIC modes */
 		vmcb->control.avic_physical_id |= AVIC_MAX_PHYSICAL_ID;
 		/* Enabling MSR intercept for x2APIC registers */
 		svm_set_x2apic_msr_interception(svm, true);
@@ -978,7 +988,6 @@ bool avic_check_apicv_inhibit_reasons(enum kvm_apicv_inhibit reason)
 			  BIT(APICV_INHIBIT_REASON_NESTED) |
 			  BIT(APICV_INHIBIT_REASON_IRQWIN) |
 			  BIT(APICV_INHIBIT_REASON_PIT_REINJ) |
-			  BIT(APICV_INHIBIT_REASON_X2APIC) |
 			  BIT(APICV_INHIBIT_REASON_BLOCKIRQ) |
 			  BIT(APICV_INHIBIT_REASON_SEV);
 
diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
index 0ec2444c342d..e04a133b98d0 100644
--- a/arch/x86/kvm/svm/svm.c
+++ b/arch/x86/kvm/svm/svm.c
@@ -4061,7 +4061,6 @@ static void svm_vcpu_after_set_cpuid(struct kvm_vcpu *vcpu)
 {
 	struct vcpu_svm *svm = to_svm(vcpu);
 	struct kvm_cpuid_entry2 *best;
-	struct kvm *kvm = vcpu->kvm;
 
 	vcpu->arch.xsaves_enabled = guest_cpuid_has(vcpu, X86_FEATURE_XSAVE) &&
 				    boot_cpu_has(X86_FEATURE_XSAVE) &&
@@ -4093,14 +4092,6 @@ static void svm_vcpu_after_set_cpuid(struct kvm_vcpu *vcpu)
 			vcpu->arch.reserved_gpa_bits &= ~(1UL << (best->ebx & 0x3f));
 	}
 
-	if (kvm_vcpu_apicv_active(vcpu)) {
-		/*
-		 * AVIC does not work with an x2APIC mode guest. If the X2APIC feature
-		 * is exposed to the guest, disable AVIC.
-		 */
-		if (guest_cpuid_has(vcpu, X86_FEATURE_X2APIC))
-			kvm_set_apicv_inhibit(kvm, APICV_INHIBIT_REASON_X2APIC);
-	}
 	init_vmcb_after_set_cpuid(vcpu);
 }
 
-- 
2.25.1