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>

diff --git a/arch/x86/kvm/svm/avic.c b/arch/x86/kvm/svm/avic.c
index 38b2f5c441e05a0008d4583f2c285007aa3131db..0797b0b9b831ffe247606cea82b7e69498a08344 100644 (file)
--- 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