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

[Stefano Garzarella]

On Fri, Mar 24, 2023 at 10:54:39AM +0800, Jason Wang wrote:
> On Thu, Mar 23, 2023 at 5:50 PM Stefano Garzarella <sgarzare@xxxxxxxxxx> wrote:
> > 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.
>
> Ok.
>
> > 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.
>
> This should be possible, I was told by Intel that their IOMMU can
> access the process page table for shared virtual memory.

Cool, we should investigate this. Do you have any pointers to their
documentation?

Thanks,
Stefano