Re: [PATCH v2 6/9] xen/gntdev: Add initial support for dma-buf UAPI

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On 06/04/2018 11:49 PM, Boris Ostrovsky wrote:
On 06/01/2018 07:41 AM, Oleksandr Andrushchenko wrote:
From: Oleksandr Andrushchenko <oleksandr_andrushchenko@xxxxxxxx>

Add UAPI and IOCTLs for dma-buf grant device driver extension:
the extension allows userspace processes and kernel modules to
use Xen backed dma-buf implementation. With this extension grant
references to the pages of an imported dma-buf can be exported
for other domain use and grant references coming from a foreign
domain can be converted into a local dma-buf for local export.
Implement basic initialization and stubs for Xen DMA buffers'
support.

It would be very helpful if people advocating for this interface
reviewed it as well.
I would also love to see their comments here ;)

Signed-off-by: Oleksandr Andrushchenko <oleksandr_andrushchenko@xxxxxxxx>
---
  drivers/xen/Kconfig         |  10 +++
  drivers/xen/Makefile        |   1 +
  drivers/xen/gntdev-dmabuf.c |  75 +++++++++++++++++++
  drivers/xen/gntdev-dmabuf.h |  41 +++++++++++
  drivers/xen/gntdev.c        | 142 ++++++++++++++++++++++++++++++++++++
  include/uapi/xen/gntdev.h   |  91 +++++++++++++++++++++++
  6 files changed, 360 insertions(+)
  create mode 100644 drivers/xen/gntdev-dmabuf.c
  create mode 100644 drivers/xen/gntdev-dmabuf.h

diff --git a/drivers/xen/Kconfig b/drivers/xen/Kconfig
index 39536ddfbce4..52d64e4b6b81 100644
--- a/drivers/xen/Kconfig
+++ b/drivers/xen/Kconfig
@@ -152,6 +152,16 @@ config XEN_GNTDEV
  	help
  	  Allows userspace processes to use grants.
+config XEN_GNTDEV_DMABUF
+	bool "Add support for dma-buf grant access device driver extension"
+	depends on XEN_GNTDEV && XEN_GRANT_DMA_ALLOC && DMA_SHARED_BUFFER

Is there a reason to have XEN_GRANT_DMA_ALLOC without XEN_GNTDEV_DMABUF?
One can use grant-table's DMA API without using dma-buf at all, e.g.
dma-buf is sort of functionality on top of DMA allocated memory.
We have a use-case for a driver domain (guest domain in fact)
backed with IOMMU and still requiring allocations created as
contiguous/DMA memory, so those buffers can be passed around to
drivers expecting DMA-only buffers.
So, IMO this is a valid use-case "to have XEN_GRANT_DMA_ALLOC
without XEN_GNTDEV_DMABUF"

+	help
+	  Allows userspace processes and kernel modules to use Xen backed
+	  dma-buf implementation. With this extension grant references to
+	  the pages of an imported dma-buf can be exported for other domain
+	  use and grant references coming from a foreign domain can be
+	  converted into a local dma-buf for local export.
+
  config XEN_GRANT_DEV_ALLOC
  	tristate "User-space grant reference allocator driver"
  	depends on XEN
diff --git a/drivers/xen/Makefile b/drivers/xen/Makefile
index 3c87b0c3aca6..33afb7b2b227 100644
--- a/drivers/xen/Makefile
+++ b/drivers/xen/Makefile
@@ -41,5 +41,6 @@ obj-$(CONFIG_XEN_PVCALLS_BACKEND)	+= pvcalls-back.o
  obj-$(CONFIG_XEN_PVCALLS_FRONTEND)	+= pvcalls-front.o
  xen-evtchn-y				:= evtchn.o
  xen-gntdev-y				:= gntdev.o
+xen-gntdev-$(CONFIG_XEN_GNTDEV_DMABUF)	+= gntdev-dmabuf.o
  xen-gntalloc-y				:= gntalloc.o
  xen-privcmd-y				:= privcmd.o
diff --git a/drivers/xen/gntdev-dmabuf.c b/drivers/xen/gntdev-dmabuf.c
new file mode 100644
index 000000000000..6bedd1387bd9
--- /dev/null
+++ b/drivers/xen/gntdev-dmabuf.c
@@ -0,0 +1,75 @@
+// SPDX-License-Identifier: GPL-2.0
+
+/*
+ * Xen dma-buf functionality for gntdev.
+ *
+ * Copyright (c) 2018 Oleksandr Andrushchenko, EPAM Systems Inc.
+ */
+
+#include <linux/slab.h>
+
+#include "gntdev-dmabuf.h"
+
+struct gntdev_dmabuf_priv {
+	int dummy;
+};
+
+/* ------------------------------------------------------------------ */
+/* DMA buffer export support.                                         */
+/* ------------------------------------------------------------------ */
+
+/* ------------------------------------------------------------------ */
+/* Implementation of wait for exported DMA buffer to be released.     */
+/* ------------------------------------------------------------------ */
Why this comment style?
Just a copy-paste from gntdev, will change to usual /*..*/

+
+int gntdev_dmabuf_exp_wait_released(struct gntdev_dmabuf_priv *priv, int fd,
+				    int wait_to_ms)
+{
+	return -EINVAL;
+}
+
+/* ------------------------------------------------------------------ */
+/* DMA buffer export support.                                         */
+/* ------------------------------------------------------------------ */
+
+int gntdev_dmabuf_exp_from_pages(struct gntdev_dmabuf_export_args *args)
+{
+	return -EINVAL;
+}
+
+/* ------------------------------------------------------------------ */
+/* DMA buffer import support.                                         */
+/* ------------------------------------------------------------------ */
+
+struct gntdev_dmabuf *
+gntdev_dmabuf_imp_to_refs(struct gntdev_dmabuf_priv *priv, struct device *dev,
+			  int fd, int count, int domid)
+{
+	return ERR_PTR(-ENOMEM);
+}
+
+u32 *gntdev_dmabuf_imp_get_refs(struct gntdev_dmabuf *gntdev_dmabuf)
+{
+	return NULL;
+}
+
+int gntdev_dmabuf_imp_release(struct gntdev_dmabuf_priv *priv, u32 fd)
+{
+	return -EINVAL;
+}
+
+struct gntdev_dmabuf_priv *gntdev_dmabuf_init(void)
+{
+	struct gntdev_dmabuf_priv *priv;
+
+	priv = kzalloc(sizeof(*priv), GFP_KERNEL);
+	if (!priv)
+		return ERR_PTR(-ENOMEM);
+
+	return priv;
+}
+
+void gntdev_dmabuf_fini(struct gntdev_dmabuf_priv *priv)
+{
+	kfree(priv);
+}
diff --git a/drivers/xen/gntdev-dmabuf.h b/drivers/xen/gntdev-dmabuf.h
new file mode 100644
index 000000000000..040b2de904ac
--- /dev/null
+++ b/drivers/xen/gntdev-dmabuf.h
@@ -0,0 +1,41 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+
+/*
+ * Xen dma-buf functionality for gntdev.
+ *
+ * Copyright (c) 2018 Oleksandr Andrushchenko, EPAM Systems Inc.
+ */
+
+#ifndef _GNTDEV_DMABUF_H
+#define _GNTDEV_DMABUF_H
+
+#include <linux/kernel.h>
+#include <linux/errno.h>
+#include <linux/types.h>
+
+struct gntdev_dmabuf_priv;
+struct gntdev_dmabuf;
+struct device;
+
+struct gntdev_dmabuf_export_args {
+	int dummy;
+};

Please define the full structure (at least what you have in the next
patch) here.
Ok, will define what I have in the next patch, but won't
initialize anything until the next patch. Will this work for you?

+
+struct gntdev_dmabuf_priv *gntdev_dmabuf_init(void);
+
+void gntdev_dmabuf_fini(struct gntdev_dmabuf_priv *priv);
+
+int gntdev_dmabuf_exp_from_pages(struct gntdev_dmabuf_export_args *args);
+
+int gntdev_dmabuf_exp_wait_released(struct gntdev_dmabuf_priv *priv, int fd,
+				    int wait_to_ms);
+
+struct gntdev_dmabuf *
+gntdev_dmabuf_imp_to_refs(struct gntdev_dmabuf_priv *priv, struct device *dev,
+			  int fd, int count, int domid);
+
+u32 *gntdev_dmabuf_imp_get_refs(struct gntdev_dmabuf *gntdev_dmabuf);
+
+int gntdev_dmabuf_imp_release(struct gntdev_dmabuf_priv *priv, u32 fd);
+
+#endif
diff --git a/drivers/xen/gntdev.c b/drivers/xen/gntdev.c
index 9813fc440c70..7d58dfb3e5e8 100644
--- a/drivers/xen/gntdev.c
+++ b/drivers/xen/gntdev.c
...

+#ifdef CONFIG_XEN_GNTDEV_DMABUF
This code belongs in gntdev-dmabuf.c.
The reason I have this code here is that it is heavily
tied to gntdev's internal functionality, e.g. map/unmap.
I do not want to extend gntdev's API, so gntdev-dmabuf can
access these. What is more dma-buf doesn't need to know about
maps done by gntdev as there is no use of that information
in gntdev-dmabuf. So, it seems more naturally to have
dma-buf's related map/unmap code where it is: in gntdev.

+/* ------------------------------------------------------------------ */
+/* DMA buffer export support.                                         */
+/* ------------------------------------------------------------------ */
+
+int gntdev_dmabuf_exp_from_refs(struct gntdev_priv *priv, int flags,
+				int count, u32 domid, u32 *refs, u32 *fd)
+{
+	/* XXX: this will need to work with gntdev's map, so leave it here. */
This doesn't help understanding what's going on (at least to me) and is
removed in the next patch. So no need for this comment.
Will remove the comment
-boris

+	*fd = -1;
+	return -EINVAL;
+}


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