Re: [PATCH v4 1/3] virtio: add dma-buf support for exported objects

[David Stevens <stevensd@chromium.org>]

On Sun, Jun 7, 2020 at 5:04 AM Michael S. Tsirkin <mst@redhat.com> wrote:
>
> On Fri, Jun 05, 2020 at 10:28:42AM +0900, David Stevens wrote:
> > On Fri, Jun 5, 2020 at 4:05 AM Michael S. Tsirkin <mst@redhat.com> wrote:
> > >
> > > On Tue, May 26, 2020 at 07:58:09PM +0900, David Stevens wrote:
> > > > This change adds a new flavor of dma-bufs that can be used by virtio
> > > > drivers to share exported objects. A virtio dma-buf can be queried by
> > > > virtio drivers to obtain the UUID which identifies the underlying
> > > > exported object.
> > > >
> > > > Signed-off-by: David Stevens <stevensd@chromium.org>
> > >
> > > Is this just for graphics? If yes I'd rather we put it in the graphics
> > > driver. We can always move it later ...
> >
> > As stated in the cover letter, this will be used by virtio-video.
> >
> > The proposed virtio-video patches: https://markmail.org/thread/p5d3k566srtdtute
> > The patch which imports these dma-bufs (slightly out of data, uses v3
> > of this patch set): https://markmail.org/thread/j4xlqaaim266qpks
> >
> > > > ---
> > > >  drivers/virtio/Makefile         |  2 +-
> > > >  drivers/virtio/virtio.c         |  6 +++
> > > >  drivers/virtio/virtio_dma_buf.c | 89 +++++++++++++++++++++++++++++++++
> > > >  include/linux/virtio.h          |  1 +
> > > >  include/linux/virtio_dma_buf.h  | 58 +++++++++++++++++++++
> > > >  5 files changed, 155 insertions(+), 1 deletion(-)
> > > >  create mode 100644 drivers/virtio/virtio_dma_buf.c
> > > >  create mode 100644 include/linux/virtio_dma_buf.h
> > > >
> > > > diff --git a/drivers/virtio/Makefile b/drivers/virtio/Makefile
> > > > index 29a1386ecc03..ecdae5b596de 100644
> > > > --- a/drivers/virtio/Makefile
> > > > +++ b/drivers/virtio/Makefile
> > > > @@ -1,5 +1,5 @@
> > > >  # SPDX-License-Identifier: GPL-2.0
> > > > -obj-$(CONFIG_VIRTIO) += virtio.o virtio_ring.o
> > > > +obj-$(CONFIG_VIRTIO) += virtio.o virtio_ring.o virtio_dma_buf.o
> > > >  obj-$(CONFIG_VIRTIO_MMIO) += virtio_mmio.o
> > > >  obj-$(CONFIG_VIRTIO_PCI) += virtio_pci.o
> > > >  virtio_pci-y := virtio_pci_modern.o virtio_pci_common.o
> > > > diff --git a/drivers/virtio/virtio.c b/drivers/virtio/virtio.c
> > > > index a977e32a88f2..5d46f0ded92d 100644
> > > > --- a/drivers/virtio/virtio.c
> > > > +++ b/drivers/virtio/virtio.c
> > > > @@ -357,6 +357,12 @@ int register_virtio_device(struct virtio_device *dev)
> > > >  }
> > > >  EXPORT_SYMBOL_GPL(register_virtio_device);
> > > >
> > > > +bool is_virtio_device(struct device *dev)
> > > > +{
> > > > +     return dev->bus == &virtio_bus;
> > > > +}
> > > > +EXPORT_SYMBOL_GPL(is_virtio_device);
> > > > +
> > > >  void unregister_virtio_device(struct virtio_device *dev)
> > > >  {
> > > >       int index = dev->index; /* save for after device release */
> > > > diff --git a/drivers/virtio/virtio_dma_buf.c b/drivers/virtio/virtio_dma_buf.c
> > > > new file mode 100644
> > > > index 000000000000..23e3399b11ed
> > > > --- /dev/null
> > > > +++ b/drivers/virtio/virtio_dma_buf.c
> > > > @@ -0,0 +1,89 @@
> > > > +// SPDX-License-Identifier: GPL-2.0-or-later
> > > > +/*
> > > > + * dma-bufs for virtio exported objects
> > > > + *
> > > > + * Copyright (C) 2020 Google, Inc.
> > > > + */
> > > > +
> > > > +#include <linux/virtio_dma_buf.h>
> > > > +
> > > > +/**
> > > > + * virtio_dma_buf_export - Creates a new dma-buf for a virtio exported object
> > > > + *
> > > > + * This wraps dma_buf_export() to allow virtio drivers to create a dma-buf
> > > > + * for an virtio exported object that can be queried by other virtio drivers
> > > > + * for the object's UUID.
> > > > + */
> > > > +struct dma_buf *virtio_dma_buf_export(
> > > > +             const struct virtio_dma_buf_export_info *virtio_exp_info)
> > > > +{
> > > > +     struct dma_buf_export_info exp_info;
> > > > +
> > > > +     if (!virtio_exp_info->ops
> > > > +             || virtio_exp_info->ops->ops.attach != &virtio_dma_buf_attach
> > > > +             || !virtio_exp_info->ops->get_uuid) {
> > > > +             return ERR_PTR(-EINVAL);
> > > > +     }
> > > > +
> > > > +     exp_info.exp_name = virtio_exp_info->exp_name;
> > > > +     exp_info.owner = virtio_exp_info->owner;
> > > > +     exp_info.ops = &virtio_exp_info->ops->ops;
> > > > +     exp_info.size = virtio_exp_info->size;
> > > > +     exp_info.flags = virtio_exp_info->flags;
> > > > +     exp_info.resv = virtio_exp_info->resv;
> > > > +     exp_info.priv = virtio_exp_info->priv;
> > > > +     BUILD_BUG_ON(sizeof(struct virtio_dma_buf_export_info)
> > > > +                  != sizeof(struct dma_buf_export_info));
> > >
> > > This is the only part that gives me pause. Why do we need this hack?
> > > What's wrong with just using dma_buf_export_info directly,
> > > and if you want the virtio ops, just using container_off?
> >
> > This approach provides a more explicit type signature and a little
> > more type safety, I think. If others don't think it's a worthwhile
> > tradeoff, I can remove it.
> >
> > -David
>
> The cost is that if dma_buf_export_info changes even slightly, we get
> weird crashes.

I'm not sure I understand what types of changes you're referring to.
As this is written, virtio-dma-buf is just another client of the
dma-buf API. If this were rewritten to use dma-buf directly, then
whatever code calls virtio_dma_buf_export would become a client of the
dma-buf API. If the semantics of existing fields in the dma-buf API
were changed and virtio-dma-buf wasn't updated, then yes, you could
get weird crashes from virtio-dma-buf. However, the same problem would
exist if virtio_dma_buf_export used dma-buf directly - changes to
dma-buf's semantics could cause weird crashes if the caller of
virtio_dma_buf_export wasn't updated properly. The only potential
source of problems I see is if virtio_dma_buf_export_info wasn't
updated properly, but virtio_dma_buf_export_info is dead simple, so I
don't know if that's really a problem.

-David