[PATCH] KVM: x86: Inhibit APICv/AVIC when changing apic id/base from the defaults

[PATCH] KVM: x86: Inhibit APICv/AVIC when changing apic id/base from the defaults

[Zeng Guang]

From: Maxim Levitsky <mlevitsk@redhat.com>

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 APICv/AVIC in case
such rare cases happen.

Signed-off-by: Maxim Levitsky <mlevitsk@redhat.com>
Signed-off-by: Zeng Guang <guang.zeng@intel.com>
---
 arch/x86/include/asm/kvm_host.h |  1 +
 arch/x86/kvm/lapic.c            | 25 +++++++++++++++++++++----
 arch/x86/kvm/lapic.h            |  8 ++++++++
 arch/x86/kvm/svm/avic.c         |  3 ++-
 arch/x86/kvm/vmx/vmx.c          |  3 ++-
 5 files changed, 34 insertions(+), 6 deletions(-)

diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index 4ff36610af6a..408cf88351dd 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -1051,6 +1051,7 @@ enum kvm_apicv_inhibit {
 	APICV_INHIBIT_REASON_BLOCKIRQ,
 	APICV_INHIBIT_REASON_ABSENT,
 	APICV_INHIBIT_REASON_SEV,
+	APICV_INHIBIT_REASON_RO_SETTINGS,
 };
 
 struct kvm_arch {
diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
index 66b0eb0bda94..f256fa974ea3 100644
--- a/arch/x86/kvm/lapic.c
+++ b/arch/x86/kvm/lapic.c
@@ -2038,6 +2038,17 @@ 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 unexpected by KVM");
+
+	kvm_set_apicv_inhibit(apic->vcpu->kvm,
+			      APICV_INHIBIT_REASON_RO_SETTINGS);
+}
+
 static int kvm_lapic_reg_write(struct kvm_lapic *apic, u32 reg, u32 val)
 {
 	int ret = 0;
@@ -2046,9 +2057,11 @@ 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 +2348,11 @@ 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)
-		pr_warn_once("APIC base relocation is unsupported by KVM");
+	     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 unexpected by KVM");
+	}
 }
 
 void kvm_apic_update_apicv(struct kvm_vcpu *vcpu)
@@ -2649,6 +2665,7 @@ 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 4e4f8a22754f..b9c406d38308 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
diff --git a/arch/x86/kvm/svm/avic.c b/arch/x86/kvm/svm/avic.c
index 421619540ff9..04a07b82b1a4 100644
--- a/arch/x86/kvm/svm/avic.c
+++ b/arch/x86/kvm/svm/avic.c
@@ -838,7 +838,8 @@ bool avic_check_apicv_inhibit_reasons(enum kvm_apicv_inhibit reason)
 			  BIT(APICV_INHIBIT_REASON_PIT_REINJ) |
 			  BIT(APICV_INHIBIT_REASON_X2APIC) |
 			  BIT(APICV_INHIBIT_REASON_BLOCKIRQ) |
-			  BIT(APICV_INHIBIT_REASON_SEV);
+			  BIT(APICV_INHIBIT_REASON_SEV) |
+			  BIT(APICV_INHIBIT_REASON_RO_SETTINGS);
 
 	return supported & BIT(reason);
 }
diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index 610355b9ccce..b31f858cacf2 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -7710,7 +7710,8 @@ static bool vmx_check_apicv_inhibit_reasons(enum kvm_apicv_inhibit reason)
 	ulong supported = BIT(APICV_INHIBIT_REASON_DISABLE) |
 			  BIT(APICV_INHIBIT_REASON_ABSENT) |
 			  BIT(APICV_INHIBIT_REASON_HYPERV) |
-			  BIT(APICV_INHIBIT_REASON_BLOCKIRQ);
+			  BIT(APICV_INHIBIT_REASON_BLOCKIRQ) |
+			  BIT(APICV_INHIBIT_REASON_RO_SETTINGS);
 
 	return supported & BIT(reason);
 }
-- 
2.27.0

Re: [PATCH] KVM: x86: Inhibit APICv/AVIC when changing apic id/base from the defaults

[Sean Christopherson]

On Fri, May 20, 2022, Zeng Guang wrote:
> From: Maxim Levitsky <mlevitsk@redhat.com>
> 
> 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 APICv/AVIC in case
> such rare cases happen.
> 
> Signed-off-by: Maxim Levitsky <mlevitsk@redhat.com>
> Signed-off-by: Zeng Guang <guang.zeng@intel.com>
> ---

All of my comments on Maxim's posting still apply[*].  At this point, unless Paolo
wants wield his hammer of authority and do a bunch of fixup, the fastest way forward
will be to wait for Maxim to respond and post patches 1-3 of that series separately.

[*] https://lore.kernel.org/all/YoZrG3n5fgMp4LQl@google.com