xen/grants: support allocating consecutive grants

For support of virtio via grant mappings in rare cases larger mappings
using consecutive grants are needed. Support those by adding a bitmap
of free grants.

As consecutive grants will be needed only in very rare cases (e.g. when
configuring a virtio device with a multi-page ring), optimize for the
normal case of non-consecutive allocations.

Signed-off-by: Juergen Gross <[e-mail]>
Reviewed-by: Boris Ostrovsky <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Juergen Gross <[e-mail]>

drivers/xen/grant-table.c

···
 
 #define pr_fmt(fmt) "xen:" KBUILD_MODNAME ": " fmt
 
+#include <linux/bitmap.h>
 #include <linux/memblock.h>
 #include <linux/sched.h>
 #include <linux/mm.h>
···
 
 static grant_ref_t **gnttab_list;
 static unsigned int nr_grant_frames;
+
+/*
+ * Handling of free grants:
+ *
+ * Free grants are in a simple list anchored in gnttab_free_head. They are
+ * linked by grant ref, the last element contains GNTTAB_LIST_END. The number
+ * of free entries is stored in gnttab_free_count.
+ * Additionally there is a bitmap of free entries anchored in
+ * gnttab_free_bitmap. This is being used for simplifying allocation of
+ * multiple consecutive grants, which is needed e.g. for support of virtio.
+ * gnttab_last_free is used to add free entries of new frames at the end of
+ * the free list.
+ * gnttab_free_tail_ptr specifies the variable which references the start
+ * of consecutive free grants ending with gnttab_last_free. This pointer is
+ * updated in a rather defensive way, in order to avoid performance hits in
+ * hot paths.
+ * All those variables are protected by gnttab_list_lock.
+ */
 static int gnttab_free_count;
-static grant_ref_t gnttab_free_head;
+static unsigned int gnttab_size;
+static grant_ref_t gnttab_free_head = GNTTAB_LIST_END;
+static grant_ref_t gnttab_last_free = GNTTAB_LIST_END;
+static grant_ref_t *gnttab_free_tail_ptr;
+static unsigned long *gnttab_free_bitmap;
 static DEFINE_SPINLOCK(gnttab_list_lock);
+
 struct grant_frames xen_auto_xlat_grant_frames;
 static unsigned int xen_gnttab_version;
 module_param_named(version, xen_gnttab_version, uint, 0);
···
 
 	ref = head = gnttab_free_head;
 	gnttab_free_count -= count;
-	while (count-- > 1)
-		head = gnttab_entry(head);
+	while (count--) {
+		bitmap_clear(gnttab_free_bitmap, head, 1);
+		if (gnttab_free_tail_ptr == __gnttab_entry(head))
+			gnttab_free_tail_ptr = &gnttab_free_head;
+		if (count)
+			head = gnttab_entry(head);
+	}
 	gnttab_free_head = gnttab_entry(head);
 	gnttab_entry(head) = GNTTAB_LIST_END;
+
+	if (!gnttab_free_count) {
+		gnttab_last_free = GNTTAB_LIST_END;
+		gnttab_free_tail_ptr = NULL;
+	}
 
 	spin_unlock_irqrestore(&gnttab_list_lock, flags);
 
 	return ref;
+}
+
+static int get_seq_entry_count(void)
+{
+	if (gnttab_last_free == GNTTAB_LIST_END || !gnttab_free_tail_ptr ||
+	    *gnttab_free_tail_ptr == GNTTAB_LIST_END)
+		return 0;
+
+	return gnttab_last_free - *gnttab_free_tail_ptr + 1;
+}
+
+/* Rebuilds the free grant list and tries to find count consecutive entries. */
+static int get_free_seq(unsigned int count)
+{
+	int ret = -ENOSPC;
+	unsigned int from, to;
+	grant_ref_t *last;
+
+	gnttab_free_tail_ptr = &gnttab_free_head;
+	last = &gnttab_free_head;
+
+	for (from = find_first_bit(gnttab_free_bitmap, gnttab_size);
+	     from < gnttab_size;
+	     from = find_next_bit(gnttab_free_bitmap, gnttab_size, to + 1)) {
+		to = find_next_zero_bit(gnttab_free_bitmap, gnttab_size,
+					from + 1);
+		if (ret < 0 && to - from >= count) {
+			ret = from;
+			bitmap_clear(gnttab_free_bitmap, ret, count);
+			from += count;
+			gnttab_free_count -= count;
+			if (from == to)
+				continue;
+		}
+
+		/*
+		 * Recreate the free list in order to have it properly sorted.
+		 * This is needed to make sure that the free tail has the maximum
+		 * possible size.
+		 */
+		while (from < to) {
+			*last = from;
+			last = __gnttab_entry(from);
+			gnttab_last_free = from;
+			from++;
+		}
+		if (to < gnttab_size)
+			gnttab_free_tail_ptr = __gnttab_entry(to - 1);
+	}
+
+	*last = GNTTAB_LIST_END;
+	if (gnttab_last_free != gnttab_size - 1)
+		gnttab_free_tail_ptr = NULL;
+
+	return ret;
+}
+
+static int get_free_entries_seq(unsigned int count)
+{
+	unsigned long flags;
+	int ret = 0;
+
+	spin_lock_irqsave(&gnttab_list_lock, flags);
+
+	if (gnttab_free_count < count) {
+		ret = gnttab_expand(count - gnttab_free_count);
+		if (ret < 0)
+			goto out;
+	}
+
+	if (get_seq_entry_count() < count) {
+		ret = get_free_seq(count);
+		if (ret >= 0)
+			goto out;
+		ret = gnttab_expand(count - get_seq_entry_count());
+		if (ret < 0)
+			goto out;
+	}
+
+	ret = *gnttab_free_tail_ptr;
+	*gnttab_free_tail_ptr = gnttab_entry(ret + count - 1);
+	gnttab_free_count -= count;
+	if (!gnttab_free_count)
+		gnttab_free_tail_ptr = NULL;
+	bitmap_clear(gnttab_free_bitmap, ret, count);
+
+ out:
+	spin_unlock_irqrestore(&gnttab_list_lock, flags);
+
+	return ret;
 }
 
 static void do_free_callbacks(void)
···
 		do_free_callbacks();
 }
 
+static void put_free_entry_locked(grant_ref_t ref)
+{
+	if (unlikely(ref < GNTTAB_NR_RESERVED_ENTRIES))
+		return;
+
+	gnttab_entry(ref) = gnttab_free_head;
+	gnttab_free_head = ref;
+	if (!gnttab_free_count)
+		gnttab_last_free = ref;
+	if (gnttab_free_tail_ptr == &gnttab_free_head)
+		gnttab_free_tail_ptr = __gnttab_entry(ref);
+	gnttab_free_count++;
+	bitmap_set(gnttab_free_bitmap, ref, 1);
+}
+
 static void put_free_entry(grant_ref_t ref)
 {
 	unsigned long flags;
 
-	if (unlikely(ref < GNTTAB_NR_RESERVED_ENTRIES))
-		return;
-
 	spin_lock_irqsave(&gnttab_list_lock, flags);
-	gnttab_entry(ref) = gnttab_free_head;
-	gnttab_free_head = ref;
-	gnttab_free_count++;
+	put_free_entry_locked(ref);
 	check_free_callbacks();
 	spin_unlock_irqrestore(&gnttab_list_lock, flags);
+}
+
+static void gnttab_set_free(unsigned int start, unsigned int n)
+{
+	unsigned int i;
+
+	for (i = start; i < start + n - 1; i++)
+		gnttab_entry(i) = i + 1;
+
+	gnttab_entry(i) = GNTTAB_LIST_END;
+	if (!gnttab_free_count) {
+		gnttab_free_head = start;
+		gnttab_free_tail_ptr = &gnttab_free_head;
+	} else {
+		gnttab_entry(gnttab_last_free) = start;
+	}
+	gnttab_free_count += n;
+	gnttab_last_free = i;
+
+	bitmap_set(gnttab_free_bitmap, start, n);
 }
 
 /*
···
 {
 	grant_ref_t ref;
 	unsigned long flags;
-	int count = 1;
-	if (head == GNTTAB_LIST_END)
-		return;
+
 	spin_lock_irqsave(&gnttab_list_lock, flags);
-	ref = head;
-	while (gnttab_entry(ref) != GNTTAB_LIST_END) {
-		ref = gnttab_entry(ref);
-		count++;
+	while (head != GNTTAB_LIST_END) {
+		ref = gnttab_entry(head);
+		put_free_entry_locked(head);
+		head = ref;
 	}
-	gnttab_entry(ref) = gnttab_free_head;
-	gnttab_free_head = head;
-	gnttab_free_count += count;
 	check_free_callbacks();
 	spin_unlock_irqrestore(&gnttab_list_lock, flags);
 }
 EXPORT_SYMBOL_GPL(gnttab_free_grant_references);
+
+void gnttab_free_grant_reference_seq(grant_ref_t head, unsigned int count)
+{
+	unsigned long flags;
+	unsigned int i;
+
+	spin_lock_irqsave(&gnttab_list_lock, flags);
+	for (i = count; i > 0; i--)
+		put_free_entry_locked(head + i - 1);
+	check_free_callbacks();
+	spin_unlock_irqrestore(&gnttab_list_lock, flags);
+}
+EXPORT_SYMBOL_GPL(gnttab_free_grant_reference_seq);
 
 int gnttab_alloc_grant_references(u16 count, grant_ref_t *head)
 {
···
 	return 0;
 }
 EXPORT_SYMBOL_GPL(gnttab_alloc_grant_references);
+
+int gnttab_alloc_grant_reference_seq(unsigned int count, grant_ref_t *first)
+{
+	int h;
+
+	if (count == 1)
+		h = get_free_entries(1);
+	else
+		h = get_free_entries_seq(count);
+
+	if (h < 0)
+		return -ENOSPC;
+
+	*first = h;
+
+	return 0;
+}
+EXPORT_SYMBOL_GPL(gnttab_alloc_grant_reference_seq);
 
 int gnttab_empty_grant_references(const grant_ref_t *private_head)
 {
···
 			goto grow_nomem;
 	}
 
+	gnttab_set_free(gnttab_size, extra_entries);
 
-	for (i = grefs_per_frame * nr_grant_frames;
-	     i < grefs_per_frame * new_nr_grant_frames - 1; i++)
-		gnttab_entry(i) = i + 1;
-
-	gnttab_entry(i) = gnttab_free_head;
-	gnttab_free_head = grefs_per_frame * nr_grant_frames;
-	gnttab_free_count += extra_entries;
+	if (!gnttab_free_tail_ptr)
+		gnttab_free_tail_ptr = __gnttab_entry(gnttab_size);
 
 	nr_grant_frames = new_nr_grant_frames;
+	gnttab_size += extra_entries;
 
 	check_free_callbacks();
 
···
 int gnttab_init(void)
 {
 	int i;
-	unsigned long max_nr_grant_frames;
+	unsigned long max_nr_grant_frames, max_nr_grefs;
 	unsigned int max_nr_glist_frames, nr_glist_frames;
-	unsigned int nr_init_grefs;
 	int ret;
 
 	gnttab_request_version();
 	max_nr_grant_frames = gnttab_max_grant_frames();
+	max_nr_grefs = max_nr_grant_frames *
+			gnttab_interface->grefs_per_grant_frame;
 	nr_grant_frames = 1;
 
 	/* Determine the maximum number of frames required for the
 	 * grant reference free list on the current hypervisor.
 	 */
-	max_nr_glist_frames = (max_nr_grant_frames *
-			       gnttab_interface->grefs_per_grant_frame / RPP);
+	max_nr_glist_frames = max_nr_grefs / RPP;
 
 	gnttab_list = kmalloc_array(max_nr_glist_frames,
 				    sizeof(grant_ref_t *),
···
 		}
 	}
 
+	gnttab_free_bitmap = bitmap_zalloc(max_nr_grefs, GFP_KERNEL);
+	if (!gnttab_free_bitmap) {
+		ret = -ENOMEM;
+		goto ini_nomem;
+	}
+
 	ret = arch_gnttab_init(max_nr_grant_frames,
 			       nr_status_frames(max_nr_grant_frames));
 	if (ret < 0)
···
 		goto ini_nomem;
 	}
 
-	nr_init_grefs = nr_grant_frames *
-			gnttab_interface->grefs_per_grant_frame;
+	gnttab_size = nr_grant_frames * gnttab_interface->grefs_per_grant_frame;
 
-	for (i = GNTTAB_NR_RESERVED_ENTRIES; i < nr_init_grefs - 1; i++)
-		gnttab_entry(i) = i + 1;
-
-	gnttab_entry(nr_init_grefs - 1) = GNTTAB_LIST_END;
-	gnttab_free_count = nr_init_grefs - GNTTAB_NR_RESERVED_ENTRIES;
-	gnttab_free_head  = GNTTAB_NR_RESERVED_ENTRIES;
+	gnttab_set_free(GNTTAB_NR_RESERVED_ENTRIES,
+			gnttab_size - GNTTAB_NR_RESERVED_ENTRIES);
 
 	printk("Grant table initialized\n");
 	return 0;
···
 	for (i--; i >= 0; i--)
 		free_page((unsigned long)gnttab_list[i]);
 	kfree(gnttab_list);
+	bitmap_free(gnttab_free_bitmap);
 	return ret;
 }
 EXPORT_SYMBOL_GPL(gnttab_init);

include/xen/grant_table.h

···
  */
 int gnttab_alloc_grant_references(u16 count, grant_ref_t *pprivate_head);
 
+int gnttab_alloc_grant_reference_seq(unsigned int count, grant_ref_t *first);
+
 void gnttab_free_grant_reference(grant_ref_t ref);
 
 void gnttab_free_grant_references(grant_ref_t head);
+
+void gnttab_free_grant_reference_seq(grant_ref_t head, unsigned int count);
 
 int gnttab_empty_grant_references(const grant_ref_t *pprivate_head);