[PATCH v2 2/2] KVM: arm64: Skip unreset vCPUs in MPIDR lookup table

[fuqiang wang]

When a VM is created with CPU hotplug support, kvm_for_each_vcpu() can
walk vCPUs that have not been reset yet. Their MPIDR_EL1 state is still
zero, which aliases vCPU0 when populating the compressed MPIDR lookup
table.

As a result, cmpidr_to_idx[0] can be overwritten with the index of an
unreset vCPU. A lookup for MPIDR 0 then returns the wrong vCPU, which
can prevent interrupts targeting vCPU0 from being delivered correctly
and make guest boot extremely slow.

Skip vCPUs whose MPIDR_EL1 value does not have the RES1 bit set.

Fixes: 5544750efd51 ("KVM: arm64: Build MPIDR to vcpu index cache at runtime")
Signed-off-by: fuqiang wang <fuqiang.wng@xxxxxxxxx>
---
 arch/arm64/include/asm/kvm_emulate.h |  9 +++++++++
 arch/arm64/kvm/arm.c                 | 14 ++++++++++++--
 2 files changed, 21 insertions(+), 2 deletions(-)

diff --git a/arch/arm64/include/asm/kvm_emulate.h b/arch/arm64/include/asm/kvm_emulate.h
index 5bf3d7e1d92c..3d2f0a8b040d 100644
--- a/arch/arm64/include/asm/kvm_emulate.h
+++ b/arch/arm64/include/asm/kvm_emulate.h
@@ -506,6 +506,15 @@ static inline unsigned long kvm_vcpu_get_mpidr_aff(struct kvm_vcpu *vcpu)
 	return __vcpu_sys_reg(vcpu, MPIDR_EL1) & MPIDR_HWID_BITMASK;
 }
 
+/*
+ * Check if vCPU MPIDR_EL1 has been reset. MPIDR_EL1.bit[31] is RES1
+ * and set during reset; a zero value indicates the vCPU is unreset.
+ */
+static inline bool kvm_vcpu_mpidr_is_reset(struct kvm_vcpu *vcpu)
+{
+	return !!(__vcpu_sys_reg(vcpu, MPIDR_EL1) & MPIDR_RES1_BITMASK);
+}
+
 static inline void kvm_vcpu_set_be(struct kvm_vcpu *vcpu)
 {
 	if (vcpu_mode_is_32bit(vcpu)) {
diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c
index 3732ee9eb0d4..fccfa97370df 100644
--- a/arch/arm64/kvm/arm.c
+++ b/arch/arm64/kvm/arm.c
@@ -887,8 +887,18 @@ static void kvm_init_mpidr_data(struct kvm *kvm)
 	data->mpidr_mask = mask;
 
 	kvm_for_each_vcpu(c, vcpu, kvm) {
-		u64 aff = kvm_vcpu_get_mpidr_aff(vcpu);
-		u16 index = kvm_mpidr_index(data, aff);
+		u64 aff;
+		u16 index;
+
+		/*
+		 * Skip vCPUs that haven't been reset yet; their MPIDR_EL1 is
+		 * zero.
+		 */
+		if (!kvm_vcpu_mpidr_is_reset(vcpu))
+			continue;
+
+		aff = kvm_vcpu_get_mpidr_aff(vcpu);
+		index = kvm_mpidr_index(data, aff);
 
 		data->cmpidr_to_idx[index] = c;
 	}
-- 
2.47.0