Re: [Qemu-devel] qdev_get_machine() can return something non-NULL but not TYPE_MACHINE

[Peter Maydell <peter.maydell@linaro.org>]

On 4 September 2017 at 20:38, Igor Mammedov <imammedo@redhat.com> wrote:
> On Mon, 4 Sep 2017 17:36:59 +0100
> Peter Maydell <peter.maydell@linaro.org> wrote:
>
>> I just got bitten by qdev_get_machine()'s behaviour on the user-only
>> emulators, where it can return something that isn't NULL and isn't
>> an instance of TYPE_MACHINE either.
> user-only shouldn't get to qdev_get_machine() at all,
> issue probably in container_get().

I need it in cpu_common_realizefn(), for
http://patchwork.ozlabs.org/patch/797940/

> I'd try to fix wrong user if possible and maybe add ifdef build failure
> to qdev_get_machine() so it would not build in user mode.

Can't ifdef, that source file is built once for all targets.

My fix (which I intend to send to the list today) is to make
it do the object_dynamic_cast() check -- if that doesn't give
a TYPE_MACHINE then we're in user mode and don't need to set
ignore_memory_transaction_failures on the cpu object anyway.

>> It looks like maybe this can happen in some cases in softmmu too,
>> judging by the way that qdev_get_hotplug_handler() does an
>> object_dynamic_cast() check that it really got back a TYPE_MACHINE.
> As I recall only bus or machine provide hotplug_handler currently,
> but it's possible to extend to other objects if we find use-case.
>
> We could do static cast to machine instead dynamic there but
> in hotplug case it will abort QEMU if error happens,
> hence dynamic check to avoid be more resilient during hotplug.
> (well, if qdev_get_machine() returns not machine during startup
> we would be screwed anyways, but that should break much earlier)

If this can't ever happen then we should be aborting; that's
the idea behind the cast macros doing assertions. I'm not
sure hotplug needs to be special here if it doesn't have
a genuine reason to think it might get back something of
the wrong type.

thanks
-- PMM