Re: [PATCH v3 8/8] vdpa_sim: add support for user VA

[Stefano Garzarella]

On Thu, Mar 23, 2023 at 11:42:07AM +0800, Jason Wang wrote:
> On Tue, Mar 21, 2023 at 11:48 PM Stefano Garzarella <sgarzare@xxxxxxxxxx> wrote:
> > The new "use_va" module parameter (default: true) is used in
> > vdpa_alloc_device() to inform the vDPA framework that the device
> > supports VA.
> >
> > vringh is initialized to use VA only when "use_va" is true and the
> > user's mm has been bound. So, only when the bus supports user VA
> > (e.g. vhost-vdpa).
> >
> > vdpasim_mm_work_fn work is used to serialize the binding to a new
> > address space when the .bind_mm callback is invoked, and unbinding
> > when the .unbind_mm callback is invoked.
> >
> > Call mmget_not_zero()/kthread_use_mm() inside the worker function
> > to pin the address space only as long as needed, following the
> > documentation of mmget() in include/linux/sched/mm.h:
> >
> >   * Never use this function to pin this address space for an
> >   * unbounded/indefinite amount of time.
>
> I wonder if everything would be simplified if we just allow the parent
> to advertise whether or not it requires the address space.
>
> Then when vhost-vDPA probes the device it can simply advertise
> use_work as true so vhost core can use get_task_mm() in this case?

IIUC set user_worker to true, it also creates the kthread in the vhost
core (but we can add another variable to avoid this).

My biggest concern is the comment in include/linux/sched/mm.h.
get_task_mm() uses mmget(), but in the documentation they advise against
pinning the address space indefinitely, so I preferred in keeping
mmgrab() in the vhost core, then call mmget_not_zero() in the worker
only when it is running.

In the future maybe mm will be used differently from parent if somehow
it is supported by iommu, so I would leave it to the parent to handle
this.

Thanks,
Stefano