[v4,12/15] KVM: SVM: Introduce hybrid-AVIC mode

Commit Message

Suthikulpanit, Suravee May 8, 2022, 2:39 a.m. UTC

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 <[e-mail]>
Reviewed-by: Maxim Levitsky <[e-mail]>
Signed-off-by: Suravee Suthikulpanit <[e-mail]>
---
 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(-)

Comments

Suthikulpanit, Suravee May 9, 2022, 11:23 a.m. UTC | #1

Maxim / Paolo,

On 5/8/2022 9:39 AM, Suravee Suthikulpanit wrote:
> 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<[e-mail]>
> Reviewed-by: Maxim Levitsky<[e-mail]>

Sorry for a typo here in the email of the "Reviewed-by" line.

Suravee

Patch

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 8e90c659de2d..ceed4b39b884 100644
--- a/arch/x86/kvm/svm/avic.c
+++ b/arch/x86/kvm/svm/avic.c
@@ -92,12 +92,22 @@  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 */
 		avic_set_x2apic_msr_interception(svm, false);
 	} else {
+		/* For xAVIC and hybrid-x2AVIC modes */
 		vmcb->control.avic_physical_id |= AVIC_MAX_PHYSICAL_ID;
 		/* Enabling MSR intercept for x2APIC registers */
 		avic_set_x2apic_msr_interception(svm, true);
@@ -999,7 +1009,6 @@  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 96a1fc1a1d1b..c0a3d4a1f3dc 100644
--- a/arch/x86/kvm/svm/svm.c
+++ b/arch/x86/kvm/svm/svm.c
@@ -4041,7 +4041,6 @@  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) &&
@@ -4073,14 +4072,6 @@  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);
 }