hw/intc/loongarch_ipi: Fix start fail with smp cpu < smp maxcpus on KVM

QEMU start failed when smp cpu < smp maxcpus , because qemu send a NULL
cpu to KVM, this patch adds a check for kvm_ipi_access_regs() to fix it.

run with '-smp 1,maxcpus=4,sockets=4,cores=1,threads=1'

we got:
Unexpected error in kvm_device_access() at ../accel/kvm/kvm-all.c:3477:
qemu-system-loongarch64: KVM_SET_DEVICE_ATTR failed: Group 1073741825 attr 0x0000000000010000: Invalid argument

Signed-off-by: Song Gao <gaosong@loongson.cn>
Reviewed-by: Bibo Mao <maobibo@loongson.cn>
Message-ID: <20250725081213.3867592-1-gaosong@loongson.cn>

hw/intc/loongarch_ipi_kvm.c

@@ -23,36 +23,41 @@ static void kvm_ipi_access_regs(void *opaque, bool write)
     LoongarchIPIState *lis = LOONGARCH_IPI(opaque);
     IPICore *core;
     uint64_t attr;
-    int cpu, fd = lis->dev_fd;
+    int i, cpu_index, fd = lis->dev_fd;
 
     if (fd == 0) {
         return;
     }
 
-    for (cpu = 0; cpu < ipi->num_cpu; cpu++) {
-        core = &ipi->cpu[cpu];
-        attr = (cpu << 16) | CORE_STATUS_OFF;
+    for (i = 0; i < ipi->num_cpu; i++) {
+        core = &ipi->cpu[i];
+        if (core->cpu == NULL) {
+            continue;
+        }
+        cpu_index = i;
+
+        attr = (cpu_index << 16) | CORE_STATUS_OFF;
         kvm_ipi_access_reg(fd, attr, &core->status, write);
 
-        attr = (cpu << 16) | CORE_EN_OFF;
+        attr = (cpu_index << 16) | CORE_EN_OFF;
         kvm_ipi_access_reg(fd, attr, &core->en, write);
 
-        attr = (cpu << 16) | CORE_SET_OFF;
+        attr = (cpu_index << 16) | CORE_SET_OFF;
         kvm_ipi_access_reg(fd, attr, &core->set, write);
 
-        attr = (cpu << 16) | CORE_CLEAR_OFF;
+        attr = (cpu_index << 16) | CORE_CLEAR_OFF;
         kvm_ipi_access_reg(fd, attr, &core->clear, write);
 
-        attr = (cpu << 16) | CORE_BUF_20;
+        attr = (cpu_index << 16) | CORE_BUF_20;
         kvm_ipi_access_reg(fd, attr, &core->buf[0], write);
 
-        attr = (cpu << 16) | CORE_BUF_28;
+        attr = (cpu_index << 16) | CORE_BUF_28;
         kvm_ipi_access_reg(fd, attr, &core->buf[2], write);
 
-        attr = (cpu << 16) | CORE_BUF_30;
+        attr = (cpu_index << 16) | CORE_BUF_30;
         kvm_ipi_access_reg(fd, attr, &core->buf[4], write);
 
-        attr = (cpu << 16) | CORE_BUF_38;
+        attr = (cpu_index << 16) | CORE_BUF_38;
         kvm_ipi_access_reg(fd, attr, &core->buf[6], write);
     }
 }