[PATCH v3 13/62] KVM: SVM: Drop redundant check in AVIC code on ID during vCPU creation

Sean Christopherson
2025-06-11T22:47:19Z

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>
Signed-off-by: Sean Christopherson <seanjc@google.com>
---
arch/x86/kvm/svm/avic.c | 37 +++++++++++++++----------------------
1 file changed, 15 insertions(+), 22 deletions(-)

diff --git a/arch/x86/kvm/svm/avic.c b/arch/x86/kvm/svm/avic.c
index f0a74b102c57..948bab48083b 100644
--- a/arch/x86/kvm/svm/avic.c
+++ b/arch/x86/kvm/svm/avic.c
@@ -256,26 +256,12 @@ 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
@@ -291,6 +277,9 @@ 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;

@@ -309,9 +298,7 @@ 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 >
@@ -319,9 +306,9 @@ 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;
}
@@ -1004,6 +991,9 @@ 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
@@ -1044,6 +1034,9 @@ 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
--
2.50.0.rc1.591.g9c95f17f64-goog