KVM: SVM: Drop redundant check in AVIC code on ID during vCPU creation

Drop avic_get_physical_id_entry()'s compatibility check on the incoming
ID, as its sole caller, avic_init_backing_page(), performs the exact same
check.  Drop avic_get_physical_id_entry() entirely as the only remaining
functionality is getting the address of the Physical ID table, and
accessing the array without an immediate bounds check is kludgy.

Opportunistically add a compile-time assertion to ensure the vcpu_id can't
result in a bounds overflow, e.g. if KVM (really) messed up a maximum
physical ID #define, as well as run-time assertions so that a NULL pointer
dereference is morphed into a safer WARN().

No functional change intended.

Tested-by: Sairaj Kodilkar <sarunkod@amd.com>
Reviewed-by: Naveen N Rao (AMD) <naveen@kernel.org>
Link: https://lore.kernel.org/r/20250611224604.313496-15-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

1 files changed, 15 insertions, 22 deletions

diff --git a/arch/x86/kvm/svm/avic.c b/arch/x86/kvm/svm/avic.c
index 38b2f5c441e0..0797b0b9b831 100644
--- a/arch/x86/kvm/svm/avic.c
+++ b/arch/x86/kvm/svm/avic.c
@@ -257,26 +257,12 @@ void avic_init_vmcb(struct vcpu_svm *svm, struct vmcb *vmcb)
 		avic_deactivate_vmcb(svm);
 }
 
-static u64 *avic_get_physical_id_entry(struct kvm_vcpu *vcpu,
-				       unsigned int index)
-{
-	u64 *avic_physical_id_table;
-	struct kvm_svm *kvm_svm = to_kvm_svm(vcpu->kvm);
-
-	if ((!x2avic_enabled && index > AVIC_MAX_PHYSICAL_ID) ||
-	    (index > X2AVIC_MAX_PHYSICAL_ID))
-		return NULL;
-
-	avic_physical_id_table = page_address(kvm_svm->avic_physical_id_table_page);
-
-	return &avic_physical_id_table[index];
-}
-
 static int avic_init_backing_page(struct kvm_vcpu *vcpu)
 {
-	u64 *entry, new_entry;
-	int id = vcpu->vcpu_id;
+	struct kvm_svm *kvm_svm = to_kvm_svm(vcpu->kvm);
 	struct vcpu_svm *svm = to_svm(vcpu);
+	u32 id = vcpu->vcpu_id;
+	u64 *table, new_entry;
 
 	/*
 	 * Inhibit AVIC if the vCPU ID is bigger than what is supported by AVIC
@@ -292,6 +278,9 @@ static int avic_init_backing_page(struct kvm_vcpu *vcpu)
 		return 0;
 	}
 
+	BUILD_BUG_ON((AVIC_MAX_PHYSICAL_ID + 1) * sizeof(*table) > PAGE_SIZE ||
+		     (X2AVIC_MAX_PHYSICAL_ID + 1) * sizeof(*table) > PAGE_SIZE);
+
 	if (WARN_ON_ONCE(!vcpu->arch.apic->regs))
 		return -EINVAL;
 
@@ -310,9 +299,7 @@ static int avic_init_backing_page(struct kvm_vcpu *vcpu)
 	}
 
 	/* Setting AVIC backing page address in the phy APIC ID table */
-	entry = avic_get_physical_id_entry(vcpu, id);
-	if (!entry)
-		return -EINVAL;
+	table = page_address(kvm_svm->avic_physical_id_table_page);
 
 	/* Note, fls64() returns the bit position, +1. */
 	BUILD_BUG_ON(__PHYSICAL_MASK_SHIFT >
@@ -320,9 +307,9 @@ static int avic_init_backing_page(struct kvm_vcpu *vcpu)
 
 	new_entry = avic_get_backing_page_address(svm) |
 		    AVIC_PHYSICAL_ID_ENTRY_VALID_MASK;
-	WRITE_ONCE(*entry, new_entry);
+	WRITE_ONCE(table[id], new_entry);
 
-	svm->avic_physical_id_cache = entry;
+	svm->avic_physical_id_cache = &table[id];
 
 	return 0;
 }
@@ -1005,6 +992,9 @@ void avic_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
 	if (WARN_ON(h_physical_id & ~AVIC_PHYSICAL_ID_ENTRY_HOST_PHYSICAL_ID_MASK))
 		return;
 
+	if (WARN_ON_ONCE(!svm->avic_physical_id_cache))
+		return;
+
 	/*
 	 * No need to update anything if the vCPU is blocking, i.e. if the vCPU
 	 * is being scheduled in after being preempted.  The CPU entries in the
@@ -1045,6 +1035,9 @@ void avic_vcpu_put(struct kvm_vcpu *vcpu)
 
 	lockdep_assert_preemption_disabled();
 
+	if (WARN_ON_ONCE(!svm->avic_physical_id_cache))
+		return;
+
 	/*
 	 * Note, reading the Physical ID entry outside of ir_list_lock is safe
 	 * as only the pCPU that has loaded (or is loading) the vCPU is allowed