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. Thanks > > Thanks, > Stefano >
