Re: [Qemu-devel] in a device or CPU instance init/realize, can I rely on something having the BQL or equivalent?

[Eduardo Habkost]

On Thu, Dec 07, 2017 at 04:53:59PM +0000, Peter Maydell wrote:
> On 7 December 2017 at 16:48, Igor Mammedov <address@hidden> wrote:
> > On Thu, 7 Dec 2017 16:05:50 +0000
> > Peter Maydell <address@hidden> wrote:
> >
> >> Hi; I'm currently writing '-cpu max' support for ARM. For that I'd
> >> like to be able to do the "probe host kernel for its supported feature
> >> set" in the CPU object's instance-init function, but I'd like to do

I don't think instance_init is appropriate for that, as
object_free(object_new(t)) must be always safe to call and free
of side-effects for all types.  Wouldn't it work if you do that
on realize?


> >> it just once and cache the answer. Can I rely on something or other
> >> having the BQL or otherwise ensuring that two threads don't run
> >> the instance_init method in parallel (eg in a hotplug situation),
> >> or do I need to create and use my own mutex to protect the cached
> >> answer data?
> > considering cached data shouldn't change during qemu lifetime it
> > shouldn't be possible for instance_init() to clash at hotplug
> > time as object_new(cpu) calls serialized within monitor/qmp loop.
> 
> Right, that was my question -- are we guaranteed that anything
> that calls object_new(cpu) does it with a lock?

object_new() itself doesn't seem to be thread-safe (see
type_initialize()), so I'm pretty sure there shouldn't be any
object_new() calls in QEMU without the BQL being held.


> 
> > But why cpu's instance_init, we could cache host's value at
> > configure_accelerator() time (could be different depending on accel).
> 
> I don't know if I need the information at configure_accelerator()
> time (if you say "-cpu cortex-a57" then we don't need to bother
> probing). The first time I know if we want to probe is when a
> "-cpu host" CPU is created.
> 
> thanks
> -- PMM

-- 
Eduardo