Michael S. Tsirkin
2020-Jun-08 06:00 UTC
[PATCH v4 1/3] virtio: add dma-buf support for exported objects
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 at 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 at 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 at 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. > > -DavidI 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. -- MST