[PATCH v2] kvm: Fix crash due to access uninitialized kvm_state

[Gavin Shan <gshan@redhat.com>]

Runs into core dump on arm64 and the backtrace extracted from the
core dump is shown as below. It's caused by accessing uninitialized
@kvm_state in kvm_flush_coalesced_mmio_buffer() due to commit 176d073029
("hw/arm/virt: Use machine_memory_devices_init()"), where the machine's
memory region is added earlier than before.

    main
    qemu_init
    configure_accelerators
    qemu_opts_foreach
    do_configure_accelerator
    accel_init_machine
    kvm_init
    virt_kvm_type
    virt_set_memmap
    machine_memory_devices_init
    memory_region_add_subregion
    memory_region_add_subregion_common
    memory_region_update_container_subregions
    memory_region_transaction_begin
    qemu_flush_coalesced_mmio_buffer
    kvm_flush_coalesced_mmio_buffer

Fix it by bailing early in kvm_flush_coalesced_mmio_buffer() on the
uninitialized @kvm_state. With this applied, no crash is observed on
arm64.

Fixes: 176d073029 ("hw/arm/virt: Use machine_memory_devices_init()")
Signed-off-by: Gavin Shan <gshan@redhat.com>
---
v2: Bail early in kvm_flush_coalesced_mmio_buffer() on the uninitialized
    @kvm_state and improved changelog        (David/Peter)
---
 accel/kvm/kvm-all.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/accel/kvm/kvm-all.c b/accel/kvm/kvm-all.c
index 373d876c05..7b3da8dc3a 100644
--- a/accel/kvm/kvm-all.c
+++ b/accel/kvm/kvm-all.c
@@ -2812,7 +2812,7 @@ void kvm_flush_coalesced_mmio_buffer(void)
 {
     KVMState *s = kvm_state;
 
-    if (s->coalesced_flush_in_progress) {
+    if (!s || s->coalesced_flush_in_progress) {
         return;
     }
 
-- 
2.41.0