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

From: Jason Wang <jasowang@redhat.com>
To: Stefano Garzarella <sgarzare@redhat.com>
Cc: virtualization@lists.linux-foundation.org, kvm@vger.kernel.org,
	stefanha@redhat.com, linux-kernel@vger.kernel.org,
	eperezma@redhat.com, "Michael S. Tsirkin" <mst@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: Mon, 27 Mar 2023 11:12:33 +0800	[thread overview]
Message-ID: <CACGkMEugnjm4U1h9JTzMh0yJ1MnsgJR=pYEO96YsTNEYLoiR-g@mail.gmail.com> (raw)
In-Reply-To: <j6d2b5zqbb7rlrem76wopsabyy344wwnkbutvacebcig5fupnu@a2xkhywajwta>

On Fri, Mar 24, 2023 at 10:43 PM Stefano Garzarella <sgarzare@redhat.com> wrote:
>
> 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@redhat.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@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.
> >
> >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?

The vtd-spec I think.

Thanks

>
> Thanks,
> Stefano
>