[16/67] KVM: SVM: Inhibit AVIC if ID is too big instead of rejecting vCPU creation

Commit Message

Sean Christopherson April 4, 2025, 7:38 p.m. UTC

Inhibit AVIC with a new "ID too big" flag if userspace creates a vCPU with
an ID that is too big, but otherwise allow vCPU creation to succeed.
Rejecting KVM_CREATE_VCPU with EINVAL violates KVM's ABI as KVM advertises
that the max vCPU ID is 4095, but disallows creating vCPUs with IDs bigger
than 254 (AVIC) or 511 (x2AVIC).

Alternatively, KVM could advertise an accurate value depending on which
AVIC mode is in use, but that wouldn't really solve the underlying problem,
e.g. would be a breaking change if KVM were to ever try and enable AVIC or
x2AVIC by default.

Cc: Maxim Levitsky <[e-mail]>
Signed-off-by: Sean Christopherson <[e-mail]>
---
 arch/x86/include/asm/kvm_host.h |  9 ++++++++-
 arch/x86/kvm/svm/avic.c         | 16 ++++++++++++++--
 arch/x86/kvm/svm/svm.h          |  3 ++-
 3 files changed, 24 insertions(+), 4 deletions(-)

Patch

diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index 54f3cf73329b..0583d8a9c8d4 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -1304,6 +1304,12 @@  enum kvm_apicv_inhibit {
 	 */
 	APICV_INHIBIT_REASON_LOGICAL_ID_ALIASED,
 
+	/*
+	 * AVIC is disabled because the vCPU's APIC ID is beyond the max
+	 * supported by AVIC/x2AVIC, i.e. the vCPU is unaddressable.
+	 */
+	APICV_INHIBIT_REASON_PHYSICAL_ID_TOO_BIG,
+
 	NR_APICV_INHIBIT_REASONS,
 };
 
@@ -1322,7 +1328,8 @@  enum kvm_apicv_inhibit {
 	__APICV_INHIBIT_REASON(IRQWIN),			\
 	__APICV_INHIBIT_REASON(PIT_REINJ),		\
 	__APICV_INHIBIT_REASON(SEV),			\
-	__APICV_INHIBIT_REASON(LOGICAL_ID_ALIASED)
+	__APICV_INHIBIT_REASON(LOGICAL_ID_ALIASED),	\
+	__APICV_INHIBIT_REASON(PHYSICAL_ID_TOO_BIG)
 
 struct kvm_arch {
 	unsigned long n_used_mmu_pages;
diff --git a/arch/x86/kvm/svm/avic.c b/arch/x86/kvm/svm/avic.c
index c8ba2ce4cfd8..ba8dfc8a12f4 100644
--- a/arch/x86/kvm/svm/avic.c
+++ b/arch/x86/kvm/svm/avic.c
@@ -286,9 +286,21 @@  static int avic_init_backing_page(struct kvm_vcpu *vcpu)
 	int id = vcpu->vcpu_id;
 	struct vcpu_svm *svm = to_svm(vcpu);
 
+	/*
+	 * Inhibit AVIC if the vCPU ID is bigger than what is supported by AVIC
+	 * hardware.  Do so immediately, i.e. don't defer the update via a
+	 * request, as avic_vcpu_load() expects to be called if and only if the
+	 * vCPU has fully initialized AVIC.  Immediately clear apicv_active,
+	 * as avic_vcpu_load() assumes avic_physical_id_cache is valid, i.e.
+	 * waiting until KVM_REQ_APICV_UPDATE is processed on the first KVM_RUN
+	 * will result in an NULL pointer deference when loading the vCPU.
+	 */
 	if ((!x2avic_enabled && id > AVIC_MAX_PHYSICAL_ID) ||
-	    (id > X2AVIC_MAX_PHYSICAL_ID))
-		return -EINVAL;
+	    (id > X2AVIC_MAX_PHYSICAL_ID)) {
+		kvm_set_apicv_inhibit(vcpu->kvm, APICV_INHIBIT_REASON_PHYSICAL_ID_TOO_BIG);
+		vcpu->arch.apic->apicv_active = false;
+		return 0;
+	}
 
 	if (WARN_ON_ONCE(!vcpu->arch.apic->regs))
 		return -EINVAL;
diff --git a/arch/x86/kvm/svm/svm.h b/arch/x86/kvm/svm/svm.h
index 1cc4e145577c..7af28802ebee 100644
--- a/arch/x86/kvm/svm/svm.h
+++ b/arch/x86/kvm/svm/svm.h
@@ -715,7 +715,8 @@  extern struct kvm_x86_nested_ops svm_nested_ops;
 	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_LOGICAL_ID_ALIASED)	\
+	BIT(APICV_INHIBIT_REASON_LOGICAL_ID_ALIASED) |	\
+	BIT(APICV_INHIBIT_REASON_PHYSICAL_ID_TOO_BIG)	\
 )
 
 bool avic_hardware_setup(void);