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

[Peter Maydell]

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
-- PMM