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[PATCH V2 2/7] xen/grants: support allocating consecutive grants



From: Juergen Gross <jgross@xxxxxxxx>

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 <jgross@xxxxxxxx>
---
Changes RFC -> V1:
   - no changes
   
Changes V1 -> V2:
   - no changes
---
 drivers/xen/grant-table.c | 238 +++++++++++++++++++++++++++++++++++++++-------
 include/xen/grant_table.h |   4 +
 2 files changed, 210 insertions(+), 32 deletions(-)

diff --git a/drivers/xen/grant-table.c b/drivers/xen/grant-table.c
index 8ccccac..1b458c0 100644
--- a/drivers/xen/grant-table.c
+++ b/drivers/xen/grant-table.c
@@ -33,6 +33,7 @@
 
 #define pr_fmt(fmt) "xen:" KBUILD_MODNAME ": " fmt
 
+#include <linux/bitmap.h>
 #include <linux/memblock.h>
 #include <linux/sched.h>
 #include <linux/mm.h>
@@ -72,9 +73,32 @@
 
 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);
@@ -170,16 +194,111 @@ static int get_free_entries(unsigned count)
 
        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;
+               }
+
+               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)
 {
        struct gnttab_free_callback *callback, *next;
@@ -206,17 +325,48 @@ static inline void check_free_callbacks(void)
                do_free_callbacks();
 }
 
-static void put_free_entry(grant_ref_t ref)
+static void put_free_entry_locked(grant_ref_t ref)
 {
-       unsigned long flags;
-       spin_lock_irqsave(&gnttab_list_lock, flags);
        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;
+
+       spin_lock_irqsave(&gnttab_list_lock, flags);
+       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);
+}
+
 /*
  * Following applies to gnttab_update_entry_v1 and gnttab_update_entry_v2.
  * Introducing a valid entry into the grant table:
@@ -448,23 +598,31 @@ void gnttab_free_grant_references(grant_ref_t head)
 {
        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)
 {
        int h = get_free_entries(count);
@@ -478,6 +636,24 @@ int gnttab_alloc_grant_references(u16 count, grant_ref_t 
*head)
 }
 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)
 {
        return (*private_head == GNTTAB_LIST_END);
@@ -570,16 +746,13 @@ static int grow_gnttab_list(unsigned int more_frames)
                        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();
 
@@ -1424,7 +1597,6 @@ int gnttab_init(void)
        int i;
        unsigned long max_nr_grant_frames;
        unsigned int max_nr_glist_frames, nr_glist_frames;
-       unsigned int nr_init_grefs;
        int ret;
 
        gnttab_request_version();
@@ -1452,6 +1624,13 @@ int gnttab_init(void)
                }
        }
 
+       i = gnttab_interface->grefs_per_grant_frame * max_nr_grant_frames;
+       gnttab_free_bitmap = bitmap_zalloc(i, 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)
@@ -1462,15 +1641,9 @@ int gnttab_init(void)
                goto ini_nomem;
        }
 
-       nr_init_grefs = nr_grant_frames *
-                       gnttab_interface->grefs_per_grant_frame;
-
-       for (i = NR_RESERVED_ENTRIES; i < nr_init_grefs - 1; i++)
-               gnttab_entry(i) = i + 1;
+       gnttab_size = nr_grant_frames * gnttab_interface->grefs_per_grant_frame;
 
-       gnttab_entry(nr_init_grefs - 1) = GNTTAB_LIST_END;
-       gnttab_free_count = nr_init_grefs - NR_RESERVED_ENTRIES;
-       gnttab_free_head  = NR_RESERVED_ENTRIES;
+       gnttab_set_free(NR_RESERVED_ENTRIES, gnttab_size - NR_RESERVED_ENTRIES);
 
        printk("Grant table initialized\n");
        return 0;
@@ -1479,6 +1652,7 @@ int gnttab_init(void)
        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);
diff --git a/include/xen/grant_table.h b/include/xen/grant_table.h
index dfd5bf3..d815e1d 100644
--- a/include/xen/grant_table.h
+++ b/include/xen/grant_table.h
@@ -129,10 +129,14 @@ int gnttab_try_end_foreign_access(grant_ref_t ref);
  */
 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);
 
 int gnttab_claim_grant_reference(grant_ref_t *pprivate_head);
-- 
2.7.4




 


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