netdev - Re: Re: [RFC v2 06/13] vduse: Introduce VDUSE

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Date:   Sun, 10 Jan 2021 18:03:52 +0800
From:   Yongji Xie <xieyongji@...edance.com>
To:     Bob Liu <bob.liu@...cle.com>
Cc:     "Michael S. Tsirkin" <mst@...hat.com>,
        Jason Wang <jasowang@...hat.com>,
        Stefan Hajnoczi <stefanha@...hat.com>, sgarzare@...hat.com,
        Parav Pandit <parav@...dia.com>, akpm@...ux-foundation.org,
        Randy Dunlap <rdunlap@...radead.org>,
        Matthew Wilcox <willy@...radead.org>, viro@...iv.linux.org.uk,
        axboe@...nel.dk, bcrl@...ck.org, corbet@....net,
        virtualization@...ts.linux-foundation.org, netdev@...r.kernel.org,
        kvm@...r.kernel.org, linux-aio@...ck.org,
        linux-fsdevel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: Re: [RFC v2 06/13] vduse: Introduce VDUSE - vDPA Device in Userspace

On Fri, Jan 8, 2021 at 9:32 PM Bob Liu <bob.liu@...cle.com> wrote:
>
> On 12/22/20 10:52 PM, Xie Yongji wrote:
> > This VDUSE driver enables implementing vDPA devices in userspace.
> > Both control path and data path of vDPA devices will be able to
> > be handled in userspace.
> >
> > In the control path, the VDUSE driver will make use of message
> > mechnism to forward the config operation from vdpa bus driver
> > to userspace. Userspace can use read()/write() to receive/reply
> > those control messages.
> >
> > In the data path, the VDUSE driver implements a MMU-based on-chip
> > IOMMU driver which supports mapping the kernel dma buffer to a
> > userspace iova region dynamically. Userspace can access those
> > iova region via mmap(). Besides, the eventfd mechanism is used to
> > trigger interrupt callbacks and receive virtqueue kicks in userspace
> >
> > Now we only support virtio-vdpa bus driver with this patch applied.
> >
> > Signed-off-by: Xie Yongji <xieyongji@...edance.com>
> > ---
> >  Documentation/driver-api/vduse.rst                 |   74 ++
> >  Documentation/userspace-api/ioctl/ioctl-number.rst |    1 +
> >  drivers/vdpa/Kconfig                               |    8 +
> >  drivers/vdpa/Makefile                              |    1 +
> >  drivers/vdpa/vdpa_user/Makefile                    |    5 +
> >  drivers/vdpa/vdpa_user/eventfd.c                   |  221 ++++
> >  drivers/vdpa/vdpa_user/eventfd.h                   |   48 +
> >  drivers/vdpa/vdpa_user/iova_domain.c               |  442 ++++++++
> >  drivers/vdpa/vdpa_user/iova_domain.h               |   93 ++
> >  drivers/vdpa/vdpa_user/vduse.h                     |   59 ++
> >  drivers/vdpa/vdpa_user/vduse_dev.c                 | 1121 ++++++++++++++++++++
> >  include/uapi/linux/vdpa.h                          |    1 +
> >  include/uapi/linux/vduse.h                         |   99 ++
> >  13 files changed, 2173 insertions(+)
> >  create mode 100644 Documentation/driver-api/vduse.rst
> >  create mode 100644 drivers/vdpa/vdpa_user/Makefile
> >  create mode 100644 drivers/vdpa/vdpa_user/eventfd.c
> >  create mode 100644 drivers/vdpa/vdpa_user/eventfd.h
> >  create mode 100644 drivers/vdpa/vdpa_user/iova_domain.c
> >  create mode 100644 drivers/vdpa/vdpa_user/iova_domain.h
> >  create mode 100644 drivers/vdpa/vdpa_user/vduse.h
> >  create mode 100644 drivers/vdpa/vdpa_user/vduse_dev.c
> >  create mode 100644 include/uapi/linux/vduse.h
> >
> > diff --git a/Documentation/driver-api/vduse.rst b/Documentation/driver-api/vduse.rst
> > new file mode 100644
> > index 000000000000..da9b3040f20a
> > --- /dev/null
> > +++ b/Documentation/driver-api/vduse.rst
> > @@ -0,0 +1,74 @@
> > +==================================
> > +VDUSE - "vDPA Device in Userspace"
> > +==================================
> > +
> > +vDPA (virtio data path acceleration) device is a device that uses a
> > +datapath which complies with the virtio specifications with vendor
> > +specific control path. vDPA devices can be both physically located on
> > +the hardware or emulated by software. VDUSE is a framework that makes it
> > +possible to implement software-emulated vDPA devices in userspace.
> > +
>
> Could you explain a bit more why need a VDUSE framework?

This can be used to implement a userspace I/O (such as storage,
network and so on) solution (virtio-based) for both container and VM.

> Software emulated vDPA devices is more likely used by debugging only when
> don't have real hardware.

I think software emulated vDPA devices should be also useful in other
cases, just like FUSE does.

> Do you think do the emulation in kernel space is not enough?
>

Doing the emulation in userspace should be more flexible.

Thanks,
Yongji