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

[Stefano Garzarella <sgarzare@redhat.com>]

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@redhat.com> 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