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From: "Michael S. Tsirkin" <mst@redhat.com>
To: Stefano Garzarella <sgarzare@redhat.com>
Cc: Jason Wang <jasowang@redhat.com>,
	virtualization@lists.linux-foundation.org, kvm@vger.kernel.org,
	stefanha@redhat.com, linux-kernel@vger.kernel.org,
	eperezma@redhat.com,
	Andrey Zhadchenko <andrey.zhadchenko@virtuozzo.com>,
	netdev@vger.kernel.org
Subject: Re: [PATCH v3 8/8] vdpa_sim: add support for user VA
Date: Thu, 23 Mar 2023 07:44:43 -0400	[thread overview]
Message-ID: <20230323074427-mutt-send-email-mst@kernel.org> (raw)
In-Reply-To: <20230323095006.jvbbdjvkdvhzcehz@sgarzare-redhat>

On Thu, Mar 23, 2023 at 10:50:06AM +0100, Stefano Garzarella 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@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

I think iommufd is supposed to handle all this detail, yes.


  reply	other threads:[~2023-03-23 11:45 UTC|newest]

Thread overview: 28+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2023-03-21 15:42 [PATCH v3 0/8] vdpa_sim: add support for user VA Stefano Garzarella
2023-03-21 15:42 ` [PATCH v3 1/8] vdpa: add bind_mm/unbind_mm callbacks Stefano Garzarella
2023-03-23  2:58   ` Jason Wang
2023-03-21 15:42 ` [PATCH v3 2/8] vhost-vdpa: use bind_mm/unbind_mm device callbacks Stefano Garzarella
2023-03-23  3:01   ` Jason Wang
2023-03-23  9:38     ` Stefano Garzarella
2023-03-21 15:42 ` [PATCH v3 3/8] vringh: replace kmap_atomic() with kmap_local_page() Stefano Garzarella
2023-03-22 10:37   ` Fabio M. De Francesco
2023-03-23  3:02   ` Jason Wang
2023-03-21 15:42 ` [PATCH v3 4/8] vringh: support VA with iotlb Stefano Garzarella
2023-03-23  3:36   ` Jason Wang
2023-03-23 10:37     ` Stefano Garzarella
2023-03-23  8:09   ` Eugenio Perez Martin
2023-03-23 10:46     ` Stefano Garzarella
2023-03-23 14:43       ` Eugenio Perez Martin
2023-03-24 14:39         ` Stefano Garzarella
2023-03-21 15:48 ` [PATCH v3 5/8] vdpa_sim: make devices agnostic for work management Stefano Garzarella
2023-03-21 15:48   ` [PATCH v3 6/8] vdpa_sim: use kthread worker Stefano Garzarella
2023-03-21 15:48   ` [PATCH v3 7/8] vdpa_sim: replace the spinlock with a mutex to protect the state Stefano Garzarella
2023-03-21 15:48   ` [PATCH v3 8/8] vdpa_sim: add support for user VA Stefano Garzarella
2023-03-23  3:42     ` Jason Wang
2023-03-23  9:50       ` Stefano Garzarella
2023-03-23 11:44         ` Michael S. Tsirkin [this message]
2023-03-24  2:54         ` Jason Wang
2023-03-24 14:43           ` Stefano Garzarella
2023-03-27  3:12             ` Jason Wang
2023-03-24  3:49     ` Jason Wang
2023-03-24 14:46       ` Stefano Garzarella

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