ALSA: xen-front: Implement handling of shared buffers

Implement shared buffer handling according to the
para-virtualized sound device protocol at xen/interface/io/sndif.h:
- manage buffer memory
- handle granted references
- handle page directories

[ Fixed missing linux/kernel.h inclusion -- tiwai ]

Signed-off-by: Oleksandr Andrushchenko <[e-mail]>
Signed-off-by: Takashi Iwai <[e-mail]>

sound/xen/Makefile

···
 
 snd_xen_front-objs := xen_snd_front.o \
 		      xen_snd_front_cfg.o \
-		      xen_snd_front_evtchnl.o
+		      xen_snd_front_evtchnl.o \
+		      xen_snd_front_shbuf.o
 
 obj-$(CONFIG_SND_XEN_FRONTEND) += snd_xen_front.o

sound/xen/xen_snd_front.c

···
 #include <linux/delay.h>
 #include <linux/module.h>
 
+#include <xen/page.h>
 #include <xen/platform_pci.h>
 #include <xen/xen.h>
 #include <xen/xenbus.h>
···
 
 	if (!xen_has_pv_devices())
 		return -ENODEV;
+
+	/* At the moment we only support case with XEN_PAGE_SIZE == PAGE_SIZE */
+	if (XEN_PAGE_SIZE != PAGE_SIZE) {
+		pr_err(XENSND_DRIVER_NAME ": different kernel and Xen page sizes are not supported: XEN_PAGE_SIZE (%lu) != PAGE_SIZE (%lu)\n",
+		       XEN_PAGE_SIZE, PAGE_SIZE);
+		return -ENODEV;
+	}
 
 	pr_info("Initialising Xen " XENSND_DRIVER_NAME " frontend driver\n");
 	return xenbus_register_frontend(&xen_driver);

sound/xen/xen_snd_front_shbuf.c

···
+// SPDX-License-Identifier: GPL-2.0 OR MIT
+
+/*
+ * Xen para-virtual sound device
+ *
+ * Copyright (C) 2016-2018 EPAM Systems Inc.
+ *
+ * Author: Oleksandr Andrushchenko <[e-mail]>
+ */
+
+#include <linux/kernel.h>
+#include <xen/xen.h>
+#include <xen/xenbus.h>
+
+#include "xen_snd_front_shbuf.h"
+
+grant_ref_t xen_snd_front_shbuf_get_dir_start(struct xen_snd_front_shbuf *buf)
+{
+	if (!buf->grefs)
+		return GRANT_INVALID_REF;
+
+	return buf->grefs[0];
+}
+
+void xen_snd_front_shbuf_clear(struct xen_snd_front_shbuf *buf)
+{
+	memset(buf, 0, sizeof(*buf));
+}
+
+void xen_snd_front_shbuf_free(struct xen_snd_front_shbuf *buf)
+{
+	int i;
+
+	if (buf->grefs) {
+		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);
+	free_pages_exact(buf->buffer, buf->buffer_sz);
+	xen_snd_front_shbuf_clear(buf);
+}
+
+/*
+ * number of grant references a page can hold with respect to the
+ * xensnd_page_directory header
+ */
+#define XENSND_NUM_GREFS_PER_PAGE ((XEN_PAGE_SIZE - \
+		offsetof(struct xensnd_page_directory, gref)) / \
+		sizeof(grant_ref_t))
+
+static void fill_page_dir(struct xen_snd_front_shbuf *buf,
+			  int num_pages_dir)
+{
+	struct xensnd_page_directory *page_dir;
+	unsigned char *ptr;
+	int i, cur_gref, grefs_left, to_copy;
+
+	ptr = buf->directory;
+	grefs_left = buf->num_grefs - num_pages_dir;
+	/*
+	 * skip grant references at the beginning, they are for pages granted
+	 * for the page directory itself
+	 */
+	cur_gref = num_pages_dir;
+	for (i = 0; i < num_pages_dir; i++) {
+		page_dir = (struct xensnd_page_directory *)ptr;
+		if (grefs_left <= XENSND_NUM_GREFS_PER_PAGE) {
+			to_copy = grefs_left;
+			page_dir->gref_dir_next_page = GRANT_INVALID_REF;
+		} else {
+			to_copy = XENSND_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 += XEN_PAGE_SIZE;
+		grefs_left -= to_copy;
+		cur_gref += to_copy;
+	}
+}
+
+static int grant_references(struct xenbus_device *xb_dev,
+			    struct xen_snd_front_shbuf *buf,
+			    int num_pages_dir, int num_pages_buffer,
+			    int num_grefs)
+{
+	grant_ref_t priv_gref_head;
+	unsigned long frame;
+	int ret, i, j, cur_ref;
+	int otherend_id;
+
+	ret = gnttab_alloc_grant_references(num_grefs, &priv_gref_head);
+	if (ret)
+		return ret;
+
+	buf->num_grefs = num_grefs;
+	otherend_id = xb_dev->otherend_id;
+	j = 0;
+
+	for (i = 0; i < num_pages_dir; i++) {
+		cur_ref = gnttab_claim_grant_reference(&priv_gref_head);
+		if (cur_ref < 0) {
+			ret = cur_ref;
+			goto fail;
+		}
+
+		frame = xen_page_to_gfn(virt_to_page(buf->directory +
+						     XEN_PAGE_SIZE * i));
+		gnttab_grant_foreign_access_ref(cur_ref, otherend_id, frame, 0);
+		buf->grefs[j++] = cur_ref;
+	}
+
+	for (i = 0; i < num_pages_buffer; i++) {
+		cur_ref = gnttab_claim_grant_reference(&priv_gref_head);
+		if (cur_ref < 0) {
+			ret = cur_ref;
+			goto fail;
+		}
+
+		frame = xen_page_to_gfn(virt_to_page(buf->buffer +
+						     XEN_PAGE_SIZE * i));
+		gnttab_grant_foreign_access_ref(cur_ref, otherend_id, frame, 0);
+		buf->grefs[j++] = cur_ref;
+	}
+
+	gnttab_free_grant_references(priv_gref_head);
+	fill_page_dir(buf, num_pages_dir);
+	return 0;
+
+fail:
+	gnttab_free_grant_references(priv_gref_head);
+	return ret;
+}
+
+static int alloc_int_buffers(struct xen_snd_front_shbuf *buf,
+			     int num_pages_dir, int num_pages_buffer,
+			     int num_grefs)
+{
+	buf->grefs = kcalloc(num_grefs, sizeof(*buf->grefs), GFP_KERNEL);
+	if (!buf->grefs)
+		return -ENOMEM;
+
+	buf->directory = kcalloc(num_pages_dir, XEN_PAGE_SIZE, GFP_KERNEL);
+	if (!buf->directory)
+		goto fail;
+
+	buf->buffer_sz = num_pages_buffer * XEN_PAGE_SIZE;
+	buf->buffer = alloc_pages_exact(buf->buffer_sz, GFP_KERNEL);
+	if (!buf->buffer)
+		goto fail;
+
+	return 0;
+
+fail:
+	kfree(buf->grefs);
+	buf->grefs = NULL;
+	kfree(buf->directory);
+	buf->directory = NULL;
+	return -ENOMEM;
+}
+
+int xen_snd_front_shbuf_alloc(struct xenbus_device *xb_dev,
+			      struct xen_snd_front_shbuf *buf,
+			      unsigned int buffer_sz)
+{
+	int num_pages_buffer, num_pages_dir, num_grefs;
+	int ret;
+
+	xen_snd_front_shbuf_clear(buf);
+
+	num_pages_buffer = DIV_ROUND_UP(buffer_sz, XEN_PAGE_SIZE);
+	/* number of pages the page directory consumes itself */
+	num_pages_dir = DIV_ROUND_UP(num_pages_buffer,
+				     XENSND_NUM_GREFS_PER_PAGE);
+	num_grefs = num_pages_buffer + num_pages_dir;
+
+	ret = alloc_int_buffers(buf, num_pages_dir,
+				num_pages_buffer, num_grefs);
+	if (ret < 0)
+		return ret;
+
+	ret = grant_references(xb_dev, buf, num_pages_dir, num_pages_buffer,
+			       num_grefs);
+	if (ret < 0)
+		return ret;
+
+	fill_page_dir(buf, num_pages_dir);
+	return 0;
+}

sound/xen/xen_snd_front_shbuf.h

···
+/* SPDX-License-Identifier: GPL-2.0 OR MIT */
+
+/*
+ * Xen para-virtual sound device
+ *
+ * Copyright (C) 2016-2018 EPAM Systems Inc.
+ *
+ * Author: Oleksandr Andrushchenko <[e-mail]>
+ */
+
+#ifndef __XEN_SND_FRONT_SHBUF_H
+#define __XEN_SND_FRONT_SHBUF_H
+
+#include <xen/grant_table.h>
+
+#include "xen_snd_front_evtchnl.h"
+
+struct xen_snd_front_shbuf {
+	int num_grefs;
+	grant_ref_t *grefs;
+	u8 *directory;
+	u8 *buffer;
+	size_t buffer_sz;
+};
+
+grant_ref_t xen_snd_front_shbuf_get_dir_start(struct xen_snd_front_shbuf *buf);
+
+int xen_snd_front_shbuf_alloc(struct xenbus_device *xb_dev,
+			      struct xen_snd_front_shbuf *buf,
+			      unsigned int buffer_sz);
+
+void xen_snd_front_shbuf_clear(struct xen_snd_front_shbuf *buf);
+
+void xen_snd_front_shbuf_free(struct xen_snd_front_shbuf *buf);
+
+#endif /* __XEN_SND_FRONT_SHBUF_H */