[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
2022-04-27T20:03:57Z

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@redhat.com>
---
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