[PATCH 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                 | 10 ++++++++++
 2 files changed, 19 insertions(+)

diff --git a/arch/arm64/include/asm/kvm_emulate.h b/arch/arm64/include/asm/kvm_emulate.h
index 5bf3d7e1d92c..8158e88aaa4e 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 MPIDR matches hardware reset value. RES1 bits are set after reset,
+ * while a zero default indicates a vCPU that has not been reset.
+ */
+static inline bool kvm_vcpu_mpidr_is_reset(u64 val)
+{
+	return (val & MIDR_RES1_MASK) == MIDR_RES1_MASK;
+}
+
 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..b47088201683 100644
--- a/arch/arm64/kvm/arm.c
+++ b/arch/arm64/kvm/arm.c
@@ -890,6 +890,16 @@ static void kvm_init_mpidr_data(struct kvm *kvm)
 		u64 aff = kvm_vcpu_get_mpidr_aff(vcpu);
 		u16 index = kvm_mpidr_index(data, aff);
 
+		/*
+		 * In the CPU hot-plug scenario, the kvm_for_each_vcpu() macro
+		 * iterates over all possible vcpus. For vcpus that are not
+		 * reset, kvm_vcpu_get_mpidr_aff() returns 0, then the index
+		 * computed by kvm_mpidr_index() will also be 0, thereby
+		 * overwriting the original value of vcpu0.
+		 */
+		if (!kvm_vcpu_mpidr_is_reset(aff))
+			continue;
+
 		data->cmpidr_to_idx[index] = c;
 	}
 
-- 
2.47.0