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Date:   Mon, 8 Jun 2020 05:05:40 -0400
From:   "Michael S. Tsirkin" <mst@redhat.com>
To:     David Stevens <stevensd@chromium.org>
Cc:     Gerd Hoffmann <kraxel@redhat.com>, David Airlie <airlied@linux.ie>,
        Daniel Vetter <daniel@ffwll.ch>,
        Sumit Semwal <sumit.semwal@linaro.org>,
        Jason Wang <jasowang@redhat.com>,
        Maarten Lankhorst <maarten.lankhorst@linux.intel.com>,
        Maxime Ripard <mripard@kernel.org>,
        Thomas Zimmermann <tzimmermann@suse.de>,
        open list <linux-kernel@vger.kernel.org>,
        ML dri-devel <dri-devel@lists.freedesktop.org>,
        "open list:VIRTIO GPU DRIVER" 
        <virtualization@lists.linux-foundation.org>,
        Linux Media Mailing List <linux-media@vger.kernel.org>,
        "moderated list:DMA BUFFER SHARING FRAMEWORK" 
        <linaro-mm-sig@lists.linaro.org>, virtio-dev@lists.oasis-open.org
Subject: Re: [PATCH v4 1/3] virtio: add dma-buf support for exported objects
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On Mon, Jun 08, 2020 at 05:32:26PM +0900, David Stevens wrote:
> On Mon, Jun 8, 2020 at 3:00 PM Michael S. Tsirkin <mst@redhat.com> wrote:
> >
> > On Mon, Jun 08, 2020 at 10:33:09AM +0900, David Stevens wrote:
> > > 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
> >
> > I think you can get weird crashes if fields in dma buf are reordered, or
> > if a field size changes.  You have a build bug catching overall struct
> > size changes but that can remain the same due do compiler padding or
> > such.
> 
> Since it's manually copying the fields instead of trying something
> clever like memcpy, I don't see how reordering the fields or changing
> the size of the fields would cause problems. Right now,
> virtio_dma_buf_export is just a regular client of dma_buf_export, no
> different than any of the other call sites in the kernel.
> 
> Overall, I don't really think that this is a problem. If someone makes
> breaking changes to the semantics of dma-buf, then they will need to
> update this call site, just like they will need to update all of the
> other call sites in the kernel. If someone adds new functionality to
> dma-buf and adds another field to dma_buf_export_info, the build bug
> is a reminder to add it to virtio_dma_buf_export_info. However, if the
> struct padding happens to work out such that the build bug doesn't
> trigger, that doesn't really matter - it just means that the new
> dma-buf feature won't be exposed by virito-dma-buf until someone needs
> it and notices that the new field is missing.
> 
> -David

Think about the reasons for the BUILD_BUG_ON being there, checking
struct sizes like this is a clear sign of something strange going on.


But really this is just unnecessary complexity anyway.

The only difference with dma_buf is get_uuid and device_attacj, isn't it?

And they are called like this:


+ */
+int virtio_dma_buf_get_uuid(struct dma_buf *dma_buf,
+                           uuid_t *uuid)
+{
+       const struct virtio_dma_buf_ops *ops = container_of(
+                       dma_buf->ops, const struct virtio_dma_buf_ops, ops);
+       
+       if (!is_virtio_dma_buf(dma_buf))
+               return -EINVAL;
+
+       return ops->get_uuid(dma_buf, uuid);
+}


So you are doing the container_of trick anyway, the extra structure
did not give us any type safety.


-- 
MST