// 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++) {
                if (likely(map_ops[cur_page].status == GNTST_okay)) {
                        buf->backend_map_handles[cur_page] =
                                map_ops[cur_page].handle;
                } else {
                        buf->backend_map_handles[cur_page] =
                                INVALID_GRANT_HANDLE;
                        if (!ret)
                                ret = -ENXIO;
                        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");