Re: [PATCH] kvm: Fix crash by initializing kvm_state early

[David Hildenbrand <david@redhat.com>]

On 31.07.23 01:48, Gavin Shan wrote:
> Runs into core dump on arm64 and the backtrace extracted from the
> core dump is shown as below. It's caused by accessing @kvm_state which
> isn't initialized at that point due to commit 176d073029 ("hw/arm/virt:
> Use machine_memory_devices_init()"), where the machine's memory region
> is added ealier than before.

s/ealier/earlier/

> 
>      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 initializing @kvm_state early. With this applied, no crash
> is observed on arm64.

Interestingly, we register memory listeners in kvm_init() after setting 
kvm_state, so in theory it should have worked fine.

But it's rather surprising that we see kvm_flush_coalesced_mmio_buffer() 
call even though we didn't even setup a listener with 
kvm_coalesce_mmio_region / kvm_uncoalesce_mmio_region.

Such a notifier-specific flush might have been better placed in the 
MemoryListener->begin() call. But that needs more thought, as 
qemu_flush_coalesced_mmio_buffer() is called from a couple of places.
> 
> Fixes: 176d073029 ("hw/arm/virt: Use machine_memory_devices_init()")
> Signed-off-by: Gavin Shan <gshan@redhat.com>
> ---
>   accel/kvm/kvm-all.c | 3 +--
>   1 file changed, 1 insertion(+), 2 deletions(-)
> 
> diff --git a/accel/kvm/kvm-all.c b/accel/kvm/kvm-all.c
> index 373d876c05..c825cba12f 100644
> --- a/accel/kvm/kvm-all.c
> +++ b/accel/kvm/kvm-all.c
> @@ -2464,6 +2464,7 @@ static int kvm_init(MachineState *ms)
>       qemu_mutex_init(&kml_slots_lock);
>   
>       s = KVM_STATE(ms->accelerator);
> +    kvm_state = s;
>   
>       /*
>        * On systems where the kernel can support different base page
> @@ -2695,8 +2696,6 @@ static int kvm_init(MachineState *ms)
>   #endif
>       }
>   
> -    kvm_state = s;
> -
>       ret = kvm_arch_init(ms, s);
>       if (ret < 0) {
>           goto err;

As an alternative, we might simply do nothing in 
kvm_flush_coalesced_mmio_buffer(), in case kvm_state is not setup yet. 
We don't have any notifier registered in that case.

Thanks!

-- 
Cheers,

David / dhildenb