[PATCH v3 21/28] KVM: x86: Inhibit APICv/AVIC if the optimized physical map is disabled

Sean Christopherson
2022-09-20T23:33:32Z

Inhibit APICv/AVIC if the optimized physical map is disabled so that KVM
KVM provides consistent APIC behavior if xAPIC IDs are aliased due to
vcpu_id being truncated and the x2APIC hotplug hack isn't enabled. If
the hotplug hack is disabled, events that are emulated by KVM will follow
architectural behavior (all matching vCPUs receive events, even if the
"match" is due to truncation), whereas APICv and AVIC will deliver events
only to the first matching vCPU, i.e. the vCPU that matches without
truncation.

Note, the "extra" inhibit is needed because KVM deliberately ignores
mismatches due to truncation when applying the APIC_ID_MODIFIED inhibit
so that large VMs (>255 vCPUs) can run with APICv/AVIC.

Fixes: TDB
Signed-off-by: Sean Christopherson <seanjc@google.com>
---
arch/x86/include/asm/kvm_host.h | 6 ++++++
arch/x86/kvm/lapic.c | 13 ++++++++++++-
arch/x86/kvm/svm/avic.c | 1 +
arch/x86/kvm/vmx/vmx.c | 1 +
4 files changed, 20 insertions(+), 1 deletion(-)

diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index 6570b5d728ef..594674eefe59 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -1097,6 +1097,12 @@ enum kvm_apicv_inhibit {
*/
APICV_INHIBIT_REASON_BLOCKIRQ,

+ /*
+ * APICv is disabled because not all vCPUs have a 1:1 mapping between
+ * APIC ID and vCPU, _and_ KVM is not applying its x2APIC hotplug hack.
+ */
+ APICV_INHIBIT_REASON_PHYSICAL_ID_ALIASED,
+
/*
* For simplicity, the APIC acceleration is inhibited
* first time either APIC ID or APIC base are changed by the guest
diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
index b344ab52556e..4db162b1f0b1 100644
--- a/arch/x86/kvm/lapic.c
+++ b/arch/x86/kvm/lapic.c
@@ -381,6 +381,16 @@ void kvm_recalculate_apic_map(struct kvm *kvm)
cluster[ldr] = apic;
}
out:
+ /*
+ * The optimized map is effectively KVM's internal version of APICv,
+ * and all unwanted aliasing that results in disabling the optimized
+ * map also applies to APICv.
+ */
+ if (!new)
+ kvm_set_apicv_inhibit(kvm, APICV_INHIBIT_REASON_PHYSICAL_ID_ALIASED);
+ else
+ kvm_clear_apicv_inhibit(kvm, APICV_INHIBIT_REASON_PHYSICAL_ID_ALIASED);
+
old = rcu_dereference_protected(kvm->arch.apic_map,
lockdep_is_held(&kvm->arch.apic_map_lock));
rcu_assign_pointer(kvm->arch.apic_map, new);
@@ -2150,7 +2160,8 @@ static void kvm_lapic_xapic_id_updated(struct kvm_lapic *apic)
/*
* Deliberately truncate the vCPU ID when detecting a modified APIC ID
* to avoid false positives if the vCPU ID, i.e. x2APIC ID, is a 32-bit
- * value.
+ * value. If the wrap/truncation results in unwatned aliasing, APICv
+ * will be inhibited as part of updating KVM's optimized APIC maps.
*/
if (kvm_xapic_id(apic) == (u8)apic->vcpu->vcpu_id)
return;
diff --git a/arch/x86/kvm/svm/avic.c b/arch/x86/kvm/svm/avic.c
index dd0e41d454a7..2908adc79ea6 100644
--- a/arch/x86/kvm/svm/avic.c
+++ b/arch/x86/kvm/svm/avic.c
@@ -965,6 +965,7 @@ bool avic_check_apicv_inhibit_reasons(enum kvm_apicv_inhibit reason)
BIT(APICV_INHIBIT_REASON_PIT_REINJ) |
BIT(APICV_INHIBIT_REASON_BLOCKIRQ) |
BIT(APICV_INHIBIT_REASON_SEV) |
+ BIT(APICV_INHIBIT_REASON_PHYSICAL_ID_ALIASED) |
BIT(APICV_INHIBIT_REASON_APIC_ID_MODIFIED) |
BIT(APICV_INHIBIT_REASON_APIC_BASE_MODIFIED) |
BIT(APICV_INHIBIT_REASON_X2APIC);
diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index b39095ef9bd7..0f9f8ae59f85 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -7963,6 +7963,7 @@ static bool vmx_check_apicv_inhibit_reasons(enum kvm_apicv_inhibit reason)
BIT(APICV_INHIBIT_REASON_ABSENT) |
BIT(APICV_INHIBIT_REASON_HYPERV) |
BIT(APICV_INHIBIT_REASON_BLOCKIRQ) |
+ BIT(APICV_INHIBIT_REASON_PHYSICAL_ID_ALIASED) |
BIT(APICV_INHIBIT_REASON_APIC_ID_MODIFIED) |
BIT(APICV_INHIBIT_REASON_APIC_BASE_MODIFIED);

--
2.37.3.968.ga6b4b080e4-goog