[PATCH V4 2/8] xen/grants: support allocating consecutive grants

From: Oleksandr Tyshchenko
Date: Thu Jun 02 2022 - 15:24:30 EST

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>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>
---
Changes RFC -> V1:
- no changes

Changes V1 -> V2:
- no changes

Changes V2 -> V3:
- rebase, move "if (unlikely(ref < GNTTAB_NR_RESERVED_ENTRIES))"
to put_free_entry_locked()
- do not overwrite "i" in gnttab_init(), introduce local max_nr_grefs
- add a comment on top of "while (from < to)" in get_free_seq()
- add Boris' R-b

Changes V3 -> V4:
- no changes
---
drivers/xen/grant-table.c | 251 +++++++++++++++++++++++++++++++++++++++-------
include/xen/grant_table.h | 4 +
2 files changed, 219 insertions(+), 36 deletions(-)

diff --git a/drivers/xen/grant-table.c b/drivers/xen/grant-table.c
index 7a18292..738029d 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>
@@ -70,9 +71,32 @@

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);
@@ -168,16 +192,116 @@ 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;
+ }
+
+ /*
+ * 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)
{
struct gnttab_free_callback *callback, *next;
@@ -204,21 +328,51 @@ 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;
-
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;
+ 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:
@@ -450,23 +604,31 @@ 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);
@@ -480,6 +642,24 @@ 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);
@@ -572,16 +752,13 @@ 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();

@@ -1424,20 +1601,20 @@ static int gnttab_expand(unsigned int req_entries)
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 *),
@@ -1454,6 +1631,12 @@ int gnttab_init(void)
}
}

+ 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)
@@ -1464,15 +1647,10 @@ int gnttab_init(void)
goto ini_nomem;
}

- nr_init_grefs = 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_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 - 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;
@@ -1481,6 +1659,7 @@ 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 527c990..e279be3 100644
--- a/include/xen/grant_table.h
+++ b/include/xen/grant_table.h
@@ -127,10 +127,14 @@ 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