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

[Gavin Shan <gshan@redhat.com>]

On 7/31/23 22:39, Peter Maydell wrote:
> On Mon, 31 Jul 2023 at 08:18, David Hildenbrand <david@redhat.com> wrote:
>>
>> 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.
> 
>> 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.
> 
> Yes, this seems better I think -- conceptually kvm_init()
> probably ought to first set up the accelerator state and
> then set kvm_state last, so that other code that looks
> at the kvm_state global either sees NULL or else a
> completely valid state, not a possibly half-initialised
> one. (We should probably also NULL the global in the
> error-exit path, though I imagine we're about to exit
> in that case.)
> 
> Is somebody able to write/test a patch for that today?
> Ideally we'd fix this for tomorrow's rc...
> 

Thanks for your comments, David and Peter. v2 was posted for a quick merge.

https://lists.nongnu.org/archive/html/qemu-arm/2023-07/msg00702.html

Thanks,
Gavin