[Xen-devel][PATCH 1/3] xen: Introduce shared buffer helpers for page directory...
Oleksandr Andrushchenko
andr2000 at gmail.com
Thu Nov 22 10:02:28 UTC 2018
From: Oleksandr Andrushchenko <oleksandr_andrushchenko at 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.
Signed-off-by: Oleksandr Andrushchenko <oleksandr_andrushchenko at 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 at 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 at 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_ */
--
2.19.1
More information about the dri-devel
mailing list