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SubjectRe: [Xen-devel][PATCH v2 1/3] xen: Introduce shared buffer helpers for page directory...
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Date
Changes in the series since v1:

- no changes to patch 1 (Xen shared code)

- ALSA: fix comment from Takashi (NULL stream->buffer)

- DRM: removed display buffer flush workaround

(proper fix is on review [1])

Thank you,

Oleksandr

On 11/30/18 9:42 AM, Oleksandr Andrushchenko wrote:
> From: Oleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>
>
> based frontends. Currently the frontends which implement
> similar code for sharing big buffers between frontend and
> backend are para-virtualized DRM and sound drivers.
> Both define the same way to share grant references of a
> data buffer with the corresponding backend with little
> differences.
>
> Move shared code into a helper module, so there is a single
> implementation of the same functionality for all.
>
> This patch introduces code which is used by sound and display
> frontend drivers without functional changes with the intention
> to remove shared code from the corresponding drivers.
>
> Signed-off-by: Oleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>
> ---
> drivers/xen/Kconfig | 3 +
> drivers/xen/Makefile | 1 +
> drivers/xen/xen-front-pgdir-shbuf.c | 553 ++++++++++++++++++++++++++++
> include/xen/xen-front-pgdir-shbuf.h | 89 +++++
> 4 files changed, 646 insertions(+)
> create mode 100644 drivers/xen/xen-front-pgdir-shbuf.c
> create mode 100644 include/xen/xen-front-pgdir-shbuf.h
>
> diff --git a/drivers/xen/Kconfig b/drivers/xen/Kconfig
> index 815b9e9bb975..838b66a9a0e7 100644
> --- a/drivers/xen/Kconfig
> +++ b/drivers/xen/Kconfig
> @@ -340,4 +340,7 @@ config XEN_SYMS
> config XEN_HAVE_VPMU
> bool
>
> +config XEN_FRONT_PGDIR_SHBUF
> + tristate
> +
> endmenu
> diff --git a/drivers/xen/Makefile b/drivers/xen/Makefile
> index 3e542f60f29f..c48927a58e10 100644
> --- a/drivers/xen/Makefile
> +++ b/drivers/xen/Makefile
> @@ -44,3 +44,4 @@ xen-gntdev-y := gntdev.o
> xen-gntdev-$(CONFIG_XEN_GNTDEV_DMABUF) += gntdev-dmabuf.o
> xen-gntalloc-y := gntalloc.o
> xen-privcmd-y := privcmd.o privcmd-buf.o
> +obj-$(CONFIG_XEN_FRONT_PGDIR_SHBUF) += xen-front-pgdir-shbuf.o
> diff --git a/drivers/xen/xen-front-pgdir-shbuf.c b/drivers/xen/xen-front-pgdir-shbuf.c
> new file mode 100644
> index 000000000000..48a658dc7ccf
> --- /dev/null
> +++ b/drivers/xen/xen-front-pgdir-shbuf.c
> @@ -0,0 +1,553 @@
> +// SPDX-License-Identifier: GPL-2.0 OR MIT
> +
> +/*
> + * Xen frontend/backend page directory based shared buffer
> + * helper module.
> + *
> + * Copyright (C) 2018 EPAM Systems Inc.
> + *
> + * Author: Oleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>
> + */
> +
> +#include <linux/module.h>
> +#include <linux/errno.h>
> +#include <linux/mm.h>
> +
> +#include <asm/xen/hypervisor.h>
> +#include <xen/balloon.h>
> +#include <xen/xen.h>
> +#include <xen/xenbus.h>
> +#include <xen/interface/io/ring.h>
> +
> +#include <xen/xen-front-pgdir-shbuf.h>
> +
> +#ifndef GRANT_INVALID_REF
> +/*
> + * FIXME: usage of grant reference 0 as invalid grant reference:
> + * grant reference 0 is valid, but never exposed to a PV driver,
> + * because of the fact it is already in use/reserved by the PV console.
> + */
> +#define GRANT_INVALID_REF 0
> +#endif
> +
> +/**
> + * This structure represents the structure of a shared page
> + * that contains grant references to the pages of the shared
> + * buffer. This structure is common to many Xen para-virtualized
> + * protocols at include/xen/interface/io/
> + */
> +struct xen_page_directory {
> + grant_ref_t gref_dir_next_page;
> + grant_ref_t gref[1]; /* Variable length */
> +};
> +
> +/**
> + * Shared buffer ops which are differently implemented
> + * depending on the allocation mode, e.g. if the buffer
> + * is allocated by the corresponding backend or frontend.
> + * Some of the operations.
> + */
> +struct xen_front_pgdir_shbuf_ops {
> + /*
> + * Calculate number of grefs required to handle this buffer,
> + * e.g. if grefs are required for page directory only or the buffer
> + * pages as well.
> + */
> + void (*calc_num_grefs)(struct xen_front_pgdir_shbuf *buf);
> +
> + /* Fill page directory according to para-virtual display protocol. */
> + void (*fill_page_dir)(struct xen_front_pgdir_shbuf *buf);
> +
> + /* Claim grant references for the pages of the buffer. */
> + int (*grant_refs_for_buffer)(struct xen_front_pgdir_shbuf *buf,
> + grant_ref_t *priv_gref_head, int gref_idx);
> +
> + /* Map grant references of the buffer. */
> + int (*map)(struct xen_front_pgdir_shbuf *buf);
> +
> + /* Unmap grant references of the buffer. */
> + int (*unmap)(struct xen_front_pgdir_shbuf *buf);
> +};
> +
> +/**
> + * Get granted reference to the very first page of the
> + * page directory. Usually this is passed to the backend,
> + * so it can find/fill the grant references to the buffer's
> + * pages.
> + *
> + * \param buf shared buffer which page directory is of interest.
> + * \return granted reference to the very first page of the
> + * page directory.
> + */
> +grant_ref_t
> +xen_front_pgdir_shbuf_get_dir_start(struct xen_front_pgdir_shbuf *buf)
> +{
> + if (!buf->grefs)
> + return GRANT_INVALID_REF;
> +
> + return buf->grefs[0];
> +}
> +EXPORT_SYMBOL_GPL(xen_front_pgdir_shbuf_get_dir_start);
> +
> +/**
> + * Map granted references of the shared buffer.
> + *
> + * Depending on the shared buffer mode of allocation
> + * (be_alloc flag) this can either do nothing (for buffers
> + * shared by the frontend itself) or map the provided granted
> + * references onto the backing storage (buf->pages).
> + *
> + * \param buf shared buffer which grants to be maped.
> + * \return zero on success or a negative number on failure.
> + */
> +int xen_front_pgdir_shbuf_map(struct xen_front_pgdir_shbuf *buf)
> +{
> + if (buf->ops && buf->ops->map)
> + return buf->ops->map(buf);
> +
> + /* No need to map own grant references. */
> + return 0;
> +}
> +EXPORT_SYMBOL_GPL(xen_front_pgdir_shbuf_map);
> +
> +/**
> + * Unmap granted references of the shared buffer.
> + *
> + * Depending on the shared buffer mode of allocation
> + * (be_alloc flag) this can either do nothing (for buffers
> + * shared by the frontend itself) or unmap the provided granted
> + * references.
> + *
> + * \param buf shared buffer which grants to be unmaped.
> + * \return zero on success or a negative number on failure.
> + */
> +int xen_front_pgdir_shbuf_unmap(struct xen_front_pgdir_shbuf *buf)
> +{
> + if (buf->ops && buf->ops->unmap)
> + return buf->ops->unmap(buf);
> +
> + /* No need to unmap own grant references. */
> + return 0;
> +}
> +EXPORT_SYMBOL_GPL(xen_front_pgdir_shbuf_unmap);
> +
> +/**
> + * Free all the resources of the shared buffer.
> + *
> + * \param buf shared buffer which resources to be freed.
> + */
> +void xen_front_pgdir_shbuf_free(struct xen_front_pgdir_shbuf *buf)
> +{
> + if (buf->grefs) {
> + int i;
> +
> + for (i = 0; i < buf->num_grefs; i++)
> + if (buf->grefs[i] != GRANT_INVALID_REF)
> + gnttab_end_foreign_access(buf->grefs[i],
> + 0, 0UL);
> + }
> + kfree(buf->grefs);
> + kfree(buf->directory);
> +}
> +EXPORT_SYMBOL_GPL(xen_front_pgdir_shbuf_free);
> +
> +/*
> + * Number of grefs a page can hold with respect to the
> + * struct xen_page_directory header.
> + */
> +#define XEN_NUM_GREFS_PER_PAGE ((PAGE_SIZE - \
> + offsetof(struct xen_page_directory, \
> + gref)) / sizeof(grant_ref_t))
> +
> +/**
> + * Get the number of pages the page directory consumes itself.
> + *
> + * \param buf shared buffer.
> + */
> +static int get_num_pages_dir(struct xen_front_pgdir_shbuf *buf)
> +{
> + return DIV_ROUND_UP(buf->num_pages, XEN_NUM_GREFS_PER_PAGE);
> +}
> +
> +/**
> + * Calculate the number of grant references needed to share the buffer
> + * and its pages when backend allocates the buffer.
> + *
> + * \param buf shared buffer.
> + */
> +static void backend_calc_num_grefs(struct xen_front_pgdir_shbuf *buf)
> +{
> + /* Only for pages the page directory consumes itself. */
> + buf->num_grefs = get_num_pages_dir(buf);
> +}
> +
> +/**
> + * Calculate the number of grant references needed to share the buffer
> + * and its pages when frontend allocates the buffer.
> + *
> + * \param buf shared buffer.
> + */
> +static void guest_calc_num_grefs(struct xen_front_pgdir_shbuf *buf)
> +{
> + /*
> + * Number of pages the page directory consumes itself
> + * plus grefs for the buffer pages.
> + */
> + buf->num_grefs = get_num_pages_dir(buf) + buf->num_pages;
> +}
> +
> +#define xen_page_to_vaddr(page) \
> + ((uintptr_t)pfn_to_kaddr(page_to_xen_pfn(page)))
> +
> +/**
> + * Unmap the buffer previously mapped with grant references
> + * provided by the backend.
> + *
> + * \param buf shared buffer.
> + * \return zero on success or a negative number on failure.
> + */
> +static int backend_unmap(struct xen_front_pgdir_shbuf *buf)
> +{
> + struct gnttab_unmap_grant_ref *unmap_ops;
> + int i, ret;
> +
> + if (!buf->pages || !buf->backend_map_handles || !buf->grefs)
> + return 0;
> +
> + unmap_ops = kcalloc(buf->num_pages, sizeof(*unmap_ops),
> + GFP_KERNEL);
> + if (!unmap_ops)
> + return -ENOMEM;
> +
> + for (i = 0; i < buf->num_pages; i++) {
> + phys_addr_t addr;
> +
> + addr = xen_page_to_vaddr(buf->pages[i]);
> + gnttab_set_unmap_op(&unmap_ops[i], addr, GNTMAP_host_map,
> + buf->backend_map_handles[i]);
> + }
> +
> + ret = gnttab_unmap_refs(unmap_ops, NULL, buf->pages,
> + buf->num_pages);
> +
> + for (i = 0; i < buf->num_pages; i++) {
> + if (unlikely(unmap_ops[i].status != GNTST_okay))
> + dev_err(&buf->xb_dev->dev,
> + "Failed to unmap page %d: %d\n",
> + i, unmap_ops[i].status);
> + }
> +
> + if (ret)
> + dev_err(&buf->xb_dev->dev,
> + "Failed to unmap grant references, ret %d", ret);
> +
> + kfree(unmap_ops);
> + kfree(buf->backend_map_handles);
> + buf->backend_map_handles = NULL;
> + return ret;
> +}
> +
> +/**
> + * Map the buffer with grant references provided by the backend.
> + *
> + * \param buf shared buffer.
> + * \return zero on success or a negative number on failure.
> + */
> +static int backend_map(struct xen_front_pgdir_shbuf *buf)
> +{
> + struct gnttab_map_grant_ref *map_ops = NULL;
> + unsigned char *ptr;
> + int ret, cur_gref, cur_dir_page, cur_page, grefs_left;
> +
> + map_ops = kcalloc(buf->num_pages, sizeof(*map_ops), GFP_KERNEL);
> + if (!map_ops)
> + return -ENOMEM;
> +
> + buf->backend_map_handles = kcalloc(buf->num_pages,
> + sizeof(*buf->backend_map_handles),
> + GFP_KERNEL);
> + if (!buf->backend_map_handles) {
> + kfree(map_ops);
> + return -ENOMEM;
> + }
> +
> + /*
> + * Read page directory to get grefs from the backend: for external
> + * buffer we only allocate buf->grefs for the page directory,
> + * so buf->num_grefs has number of pages in the page directory itself.
> + */
> + ptr = buf->directory;
> + grefs_left = buf->num_pages;
> + cur_page = 0;
> + for (cur_dir_page = 0; cur_dir_page < buf->num_grefs; cur_dir_page++) {
> + struct xen_page_directory *page_dir =
> + (struct xen_page_directory *)ptr;
> + int to_copy = XEN_NUM_GREFS_PER_PAGE;
> +
> + if (to_copy > grefs_left)
> + to_copy = grefs_left;
> +
> + for (cur_gref = 0; cur_gref < to_copy; cur_gref++) {
> + phys_addr_t addr;
> +
> + addr = xen_page_to_vaddr(buf->pages[cur_page]);
> + gnttab_set_map_op(&map_ops[cur_page], addr,
> + GNTMAP_host_map,
> + page_dir->gref[cur_gref],
> + buf->xb_dev->otherend_id);
> + cur_page++;
> + }
> +
> + grefs_left -= to_copy;
> + ptr += PAGE_SIZE;
> + }
> + ret = gnttab_map_refs(map_ops, NULL, buf->pages, buf->num_pages);
> +
> + /* Save handles even if error, so we can unmap. */
> + for (cur_page = 0; cur_page < buf->num_pages; cur_page++) {
> + buf->backend_map_handles[cur_page] = map_ops[cur_page].handle;
> + if (unlikely(map_ops[cur_page].status != GNTST_okay))
> + dev_err(&buf->xb_dev->dev,
> + "Failed to map page %d: %d\n",
> + cur_page, map_ops[cur_page].status);
> + }
> +
> + if (ret) {
> + dev_err(&buf->xb_dev->dev,
> + "Failed to map grant references, ret %d", ret);
> + backend_unmap(buf);
> + }
> +
> + kfree(map_ops);
> + return ret;
> +}
> +
> +/**
> + * Fill page directory with grant references to the pages of the
> + * page directory itself.
> + *
> + * The grant references to the buffer pages are provided by the
> + * backend in this case.
> + *
> + * \param buf shared buffer.
> + */
> +static void backend_fill_page_dir(struct xen_front_pgdir_shbuf *buf)
> +{
> + struct xen_page_directory *page_dir;
> + unsigned char *ptr;
> + int i, num_pages_dir;
> +
> + ptr = buf->directory;
> + num_pages_dir = get_num_pages_dir(buf);
> +
> + /* Fill only grefs for the page directory itself. */
> + for (i = 0; i < num_pages_dir - 1; i++) {
> + page_dir = (struct xen_page_directory *)ptr;
> +
> + page_dir->gref_dir_next_page = buf->grefs[i + 1];
> + ptr += PAGE_SIZE;
> + }
> + /* Last page must say there is no more pages. */
> + page_dir = (struct xen_page_directory *)ptr;
> + page_dir->gref_dir_next_page = GRANT_INVALID_REF;
> +}
> +
> +/**
> + * Fill page directory with grant references to the pages of the
> + * page directory and the buffer we share with the backend.
> + *
> + * \param buf shared buffer.
> + */
> +static void guest_fill_page_dir(struct xen_front_pgdir_shbuf *buf)
> +{
> + unsigned char *ptr;
> + int cur_gref, grefs_left, to_copy, i, num_pages_dir;
> +
> + ptr = buf->directory;
> + num_pages_dir = get_num_pages_dir(buf);
> +
> + /*
> + * While copying, skip grefs at start, they are for pages
> + * granted for the page directory itself.
> + */
> + cur_gref = num_pages_dir;
> + grefs_left = buf->num_pages;
> + for (i = 0; i < num_pages_dir; i++) {
> + struct xen_page_directory *page_dir =
> + (struct xen_page_directory *)ptr;
> +
> + if (grefs_left <= XEN_NUM_GREFS_PER_PAGE) {
> + to_copy = grefs_left;
> + page_dir->gref_dir_next_page = GRANT_INVALID_REF;
> + } else {
> + to_copy = XEN_NUM_GREFS_PER_PAGE;
> + page_dir->gref_dir_next_page = buf->grefs[i + 1];
> + }
> + memcpy(&page_dir->gref, &buf->grefs[cur_gref],
> + to_copy * sizeof(grant_ref_t));
> + ptr += PAGE_SIZE;
> + grefs_left -= to_copy;
> + cur_gref += to_copy;
> + }
> +}
> +
> +/**
> + * Grant references to the frontend's buffer pages.
> + *
> + * These will be shared with the backend, so it can
> + * access the buffer's data.
> + *
> + * \param buf shared buffer.
> + * \return zero on success or a negative number on failure.
> + */
> +static int guest_grant_refs_for_buffer(struct xen_front_pgdir_shbuf *buf,
> + grant_ref_t *priv_gref_head,
> + int gref_idx)
> +{
> + int i, cur_ref, otherend_id;
> +
> + otherend_id = buf->xb_dev->otherend_id;
> + for (i = 0; i < buf->num_pages; i++) {
> + cur_ref = gnttab_claim_grant_reference(priv_gref_head);
> + if (cur_ref < 0)
> + return cur_ref;
> +
> + gnttab_grant_foreign_access_ref(cur_ref, otherend_id,
> + xen_page_to_gfn(buf->pages[i]),
> + 0);
> + buf->grefs[gref_idx++] = cur_ref;
> + }
> + return 0;
> +}
> +
> +/**
> + * Grant all the references needed to share the buffer.
> + *
> + * Grant references to the page directory pages and, if
> + * needed, also to the pages of the shared buffer data.
> + *
> + * \param buf shared buffer.
> + * \return zero on success or a negative number on failure.
> + */
> +static int grant_references(struct xen_front_pgdir_shbuf *buf)
> +{
> + grant_ref_t priv_gref_head;
> + int ret, i, j, cur_ref;
> + int otherend_id, num_pages_dir;
> +
> + ret = gnttab_alloc_grant_references(buf->num_grefs, &priv_gref_head);
> + if (ret < 0) {
> + dev_err(&buf->xb_dev->dev,
> + "Cannot allocate grant references\n");
> + return ret;
> + }
> +
> + otherend_id = buf->xb_dev->otherend_id;
> + j = 0;
> + num_pages_dir = get_num_pages_dir(buf);
> + for (i = 0; i < num_pages_dir; i++) {
> + unsigned long frame;
> +
> + cur_ref = gnttab_claim_grant_reference(&priv_gref_head);
> + if (cur_ref < 0)
> + return cur_ref;
> +
> + frame = xen_page_to_gfn(virt_to_page(buf->directory +
> + PAGE_SIZE * i));
> + gnttab_grant_foreign_access_ref(cur_ref, otherend_id, frame, 0);
> + buf->grefs[j++] = cur_ref;
> + }
> +
> + if (buf->ops->grant_refs_for_buffer) {
> + ret = buf->ops->grant_refs_for_buffer(buf, &priv_gref_head, j);
> + if (ret)
> + return ret;
> + }
> +
> + gnttab_free_grant_references(priv_gref_head);
> + return 0;
> +}
> +
> +/**
> + * Allocate all required structures to mange shared buffer.
> + *
> + * \param buf shared buffer.
> + * \return zero on success or a negative number on failure.
> + */
> +static int alloc_storage(struct xen_front_pgdir_shbuf *buf)
> +{
> + buf->grefs = kcalloc(buf->num_grefs, sizeof(*buf->grefs), GFP_KERNEL);
> + if (!buf->grefs)
> + return -ENOMEM;
> +
> + buf->directory = kcalloc(get_num_pages_dir(buf), PAGE_SIZE, GFP_KERNEL);
> + if (!buf->directory)
> + return -ENOMEM;
> +
> + return 0;
> +}
> +
> +/*
> + * For backend allocated buffers we don't need grant_refs_for_buffer
> + * as those grant references are allocated at backend side.
> + */
> +static const struct xen_front_pgdir_shbuf_ops backend_ops = {
> + .calc_num_grefs = backend_calc_num_grefs,
> + .fill_page_dir = backend_fill_page_dir,
> + .map = backend_map,
> + .unmap = backend_unmap
> +};
> +
> +/*
> + * For locally granted references we do not need to map/unmap
> + * the references.
> + */
> +static const struct xen_front_pgdir_shbuf_ops local_ops = {
> + .calc_num_grefs = guest_calc_num_grefs,
> + .fill_page_dir = guest_fill_page_dir,
> + .grant_refs_for_buffer = guest_grant_refs_for_buffer,
> +};
> +
> +/**
> + * Allocate a new instance of a shared buffer.
> + *
> + * \param cfg configuration to be used while allocating a new shared buffer.
> + * \return zero on success or a negative number on failure.
> + */
> +int xen_front_pgdir_shbuf_alloc(struct xen_front_pgdir_shbuf_cfg *cfg)
> +{
> + struct xen_front_pgdir_shbuf *buf = cfg->pgdir;
> + int ret;
> +
> + if (cfg->be_alloc)
> + buf->ops = &backend_ops;
> + else
> + buf->ops = &local_ops;
> + buf->xb_dev = cfg->xb_dev;
> + buf->num_pages = cfg->num_pages;
> + buf->pages = cfg->pages;
> +
> + buf->ops->calc_num_grefs(buf);
> +
> + ret = alloc_storage(buf);
> + if (ret)
> + goto fail;
> +
> + ret = grant_references(buf);
> + if (ret)
> + goto fail;
> +
> + buf->ops->fill_page_dir(buf);
> +
> + return 0;
> +
> +fail:
> + xen_front_pgdir_shbuf_free(buf);
> + return ret;
> +}
> +EXPORT_SYMBOL_GPL(xen_front_pgdir_shbuf_alloc);
> +
> +MODULE_DESCRIPTION("Xen frontend/backend page directory based "
> + "shared buffer handling");
> +MODULE_AUTHOR("Oleksandr Andrushchenko");
> +MODULE_LICENSE("GPL");
> diff --git a/include/xen/xen-front-pgdir-shbuf.h b/include/xen/xen-front-pgdir-shbuf.h
> new file mode 100644
> index 000000000000..150ef7ec51ec
> --- /dev/null
> +++ b/include/xen/xen-front-pgdir-shbuf.h
> @@ -0,0 +1,89 @@
> +/* SPDX-License-Identifier: GPL-2.0 OR MIT */
> +
> +/*
> + * Xen frontend/backend page directory based shared buffer
> + * helper module.
> + *
> + * Copyright (C) 2018 EPAM Systems Inc.
> + *
> + * Author: Oleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>
> + */
> +
> +#ifndef __XEN_FRONT_PGDIR_SHBUF_H_
> +#define __XEN_FRONT_PGDIR_SHBUF_H_
> +
> +#include <linux/kernel.h>
> +
> +#include <xen/grant_table.h>
> +
> +struct xen_front_pgdir_shbuf_ops;
> +
> +struct xen_front_pgdir_shbuf {
> + /*
> + * Number of references granted for the backend use:
> + *
> + * - for frontend allocated/imported buffers this holds the number
> + * of grant references for the page directory and the pages
> + * of the buffer
> + *
> + * - for the buffer provided by the backend this only holds the number
> + * of grant references for the page directory itself as grant
> + * references for the buffer will be provided by the backend.
> + */
> + int num_grefs;
> + grant_ref_t *grefs;
> + /* Page directory backing storage. */
> + u8 *directory;
> +
> + /*
> + * Number of pages for the shared buffer itself (excluding the page
> + * directory).
> + */
> + int num_pages;
> + /*
> + * Backing storage of the shared buffer: these are the pages being
> + * shared.
> + */
> + struct page **pages;
> +
> + struct xenbus_device *xb_dev;
> +
> + /* These are the ops used internally depending on be_alloc mode. */
> + const struct xen_front_pgdir_shbuf_ops *ops;
> +
> + /* Xen map handles for the buffer allocated by the backend. */
> + grant_handle_t *backend_map_handles;
> +};
> +
> +struct xen_front_pgdir_shbuf_cfg {
> + struct xenbus_device *xb_dev;
> +
> + /* Number of pages of the buffer backing storage. */
> + int num_pages;
> + /* Pages of the buffer to be shared. */
> + struct page **pages;
> +
> + /*
> + * This is allocated outside because there are use-cases when
> + * the buffer structure is allocated as a part of a bigger one.
> + */
> + struct xen_front_pgdir_shbuf *pgdir;
> + /*
> + * Mode of grant reference sharing: if set then backend will share
> + * grant references to the buffer with the frontend.
> + */
> + int be_alloc;
> +};
> +
> +int xen_front_pgdir_shbuf_alloc(struct xen_front_pgdir_shbuf_cfg *cfg);
> +
> +grant_ref_t
> +xen_front_pgdir_shbuf_get_dir_start(struct xen_front_pgdir_shbuf *buf);
> +
> +int xen_front_pgdir_shbuf_map(struct xen_front_pgdir_shbuf *buf);
> +
> +int xen_front_pgdir_shbuf_unmap(struct xen_front_pgdir_shbuf *buf);
> +
> +void xen_front_pgdir_shbuf_free(struct xen_front_pgdir_shbuf *buf);
> +
> +#endif /* __XEN_FRONT_PGDIR_SHBUF_H_ */

[1] https://lkml.org/lkml/2018/11/27/811

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