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

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Date:   Fri, 24 Mar 2023 10:54:39 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     Stefano Garzarella <sgarzare@...hat.com>
Cc:     virtualization@...ts.linux-foundation.org, kvm@...r.kernel.org,
        stefanha@...hat.com, linux-kernel@...r.kernel.org,
        eperezma@...hat.com, "Michael S. Tsirkin" <mst@...hat.com>,
        Andrey Zhadchenko <andrey.zhadchenko@...tuozzo.com>,
        netdev@...r.kernel.org
Subject: Re: [PATCH v3 8/8] vdpa_sim: add support for user VA

On Thu, Mar 23, 2023 at 5:50 PM Stefano Garzarella <sgarzare@...hat.com> 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@...hat.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.

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
>