[RFC PATCH v3 02/19] KVM: x86: inhibit APICv/AVIC when the guest and/or host changes apic id/base from the defaults.

[Maxim Levitsky]

Neither of these settings should be changed by the guest and it is
a burden to support it in the acceleration code, so just inhibit
it instead.

Also add a boolean 'apic_id_changed' to indicate if apic id ever changed.

Signed-off-by: Maxim Levitsky <mlevitsk@xxxxxxxxxx>
---
 arch/x86/include/asm/kvm_host.h |  3 +++
 arch/x86/kvm/lapic.c            | 25 ++++++++++++++++++++++---
 arch/x86/kvm/lapic.h            |  8 ++++++++
 3 files changed, 33 insertions(+), 3 deletions(-)

diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index 63eae00625bda..636df87542555 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -1070,6 +1070,8 @@ enum kvm_apicv_inhibit {
 	APICV_INHIBIT_REASON_ABSENT,
 	/* AVIC is disabled because SEV doesn't support it */
 	APICV_INHIBIT_REASON_SEV,
+	/* APIC ID and/or APIC base was changed by the guest */
+	APICV_INHIBIT_REASON_RO_SETTINGS,
 };
 
 struct kvm_arch {
@@ -1258,6 +1260,7 @@ struct kvm_arch {
 	hpa_t	hv_root_tdp;
 	spinlock_t hv_root_tdp_lock;
 #endif
+	bool apic_id_changed;
 };
 
 struct kvm_vm_stat {
diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
index 66b0eb0bda94e..8996675b3ef4c 100644
--- a/arch/x86/kvm/lapic.c
+++ b/arch/x86/kvm/lapic.c
@@ -2038,6 +2038,19 @@ static void apic_manage_nmi_watchdog(struct kvm_lapic *apic, u32 lvt0_val)
 	}
 }
 
+static void kvm_lapic_check_initial_apic_id(struct kvm_lapic *apic)
+{
+	if (kvm_apic_has_initial_apic_id(apic))
+		return;
+
+	pr_warn_once("APIC ID change is unsupported by KVM");
+
+	kvm_set_apicv_inhibit(apic->vcpu->kvm,
+			APICV_INHIBIT_REASON_RO_SETTINGS);
+
+	apic->vcpu->kvm->arch.apic_id_changed = true;
+}
+
 static int kvm_lapic_reg_write(struct kvm_lapic *apic, u32 reg, u32 val)
 {
 	int ret = 0;
@@ -2046,9 +2059,11 @@ static int kvm_lapic_reg_write(struct kvm_lapic *apic, u32 reg, u32 val)
 
 	switch (reg) {
 	case APIC_ID:		/* Local APIC ID */
-		if (!apic_x2apic_mode(apic))
+		if (!apic_x2apic_mode(apic)) {
+
 			kvm_apic_set_xapic_id(apic, val >> 24);
-		else
+			kvm_lapic_check_initial_apic_id(apic);
+		} else
 			ret = 1;
 		break;
 
@@ -2335,8 +2350,11 @@ void kvm_lapic_set_base(struct kvm_vcpu *vcpu, u64 value)
 			     MSR_IA32_APICBASE_BASE;
 
 	if ((value & MSR_IA32_APICBASE_ENABLE) &&
-	     apic->base_address != APIC_DEFAULT_PHYS_BASE)
+	     apic->base_address != APIC_DEFAULT_PHYS_BASE) {
+		kvm_set_apicv_inhibit(apic->vcpu->kvm,
+				APICV_INHIBIT_REASON_RO_SETTINGS);
 		pr_warn_once("APIC base relocation is unsupported by KVM");
+	}
 }
 
 void kvm_apic_update_apicv(struct kvm_vcpu *vcpu)
@@ -2649,6 +2667,7 @@ static int kvm_apic_state_fixup(struct kvm_vcpu *vcpu,
 		}
 	}
 
+	kvm_lapic_check_initial_apic_id(vcpu->arch.apic);
 	return 0;
 }
 
diff --git a/arch/x86/kvm/lapic.h b/arch/x86/kvm/lapic.h
index 4e4f8a22754f9..b9c406d383080 100644
--- a/arch/x86/kvm/lapic.h
+++ b/arch/x86/kvm/lapic.h
@@ -252,4 +252,12 @@ static inline u8 kvm_xapic_id(struct kvm_lapic *apic)
 	return kvm_lapic_get_reg(apic, APIC_ID) >> 24;
 }
 
+static inline bool kvm_apic_has_initial_apic_id(struct kvm_lapic *apic)
+{
+	if (apic_x2apic_mode(apic))
+		return true;
+
+	return kvm_xapic_id(apic) == apic->vcpu->vcpu_id;
+}
+
 #endif
-- 
2.26.3