[Qemu-devel] [PATCH 03/55] Teach block/vdi about "discarded" (no longer allocated) blocks

[Kevin Wolf <kwolf@redhat.com>]

From: Eric Sunshine <sunshine@sunshineco.com>

An entry in the VDI block map will hold an offset to the actual block if
the block is allocated, or one of two specially-interpreted values if
not allocated. Using VirtualBox terminology, value VDI_IMAGE_BLOCK_FREE
(0xffffffff) represents a never-allocated block (semantically arbitrary
content).  VDI_IMAGE_BLOCK_ZERO (0xfffffffe) represents a "discarded"
block (semantically zero-filled).  block/vdi knows only about
VDI_IMAGE_BLOCK_FREE.  Teach it about VDI_IMAGE_BLOCK_ZERO.

Signed-off-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
---
 block/vdi.c |   23 ++++++++++++++---------
 1 files changed, 14 insertions(+), 9 deletions(-)

diff --git a/block/vdi.c b/block/vdi.c
index 883046d..523a640 100644
--- a/block/vdi.c
+++ b/block/vdi.c
@@ -114,8 +114,13 @@ void uuid_unparse(const uuid_t uu, char *out);
  */
 #define VDI_TEXT "<<< QEMU VM Virtual Disk Image >>>\n"
 
-/* Unallocated blocks use this index (no need to convert endianness). */
-#define VDI_UNALLOCATED UINT32_MAX
+/* A never-allocated block; semantically arbitrary content. */
+#define VDI_UNALLOCATED 0xffffffffU
+
+/* A discarded (no longer allocated) block; semantically zero-filled. */
+#define VDI_DISCARDED   0xfffffffeU
+
+#define VDI_IS_ALLOCATED(X) ((X) < VDI_DISCARDED)
 
 #if !defined(CONFIG_UUID)
 void uuid_generate(uuid_t out)
@@ -307,10 +312,10 @@ static int vdi_check(BlockDriverState *bs, BdrvCheckResult *res)
     /* Check block map and value of blocks_allocated. */
     for (block = 0; block < s->header.blocks_in_image; block++) {
         uint32_t bmap_entry = le32_to_cpu(s->bmap[block]);
-        if (bmap_entry != VDI_UNALLOCATED) {
+        if (VDI_IS_ALLOCATED(bmap_entry)) {
             if (bmap_entry < s->header.blocks_in_image) {
                 blocks_allocated++;
-                if (bmap[bmap_entry] == VDI_UNALLOCATED) {
+                if (!VDI_IS_ALLOCATED(bmap[bmap_entry])) {
                     bmap[bmap_entry] = bmap_entry;
                 } else {
                     fprintf(stderr, "ERROR: block index %" PRIu32
@@ -472,7 +477,7 @@ static int vdi_is_allocated(BlockDriverState *bs, int64_t sector_num,
         n_sectors = nb_sectors;
     }
     *pnum = n_sectors;
-    return bmap_entry != VDI_UNALLOCATED;
+    return VDI_IS_ALLOCATED(bmap_entry);
 }
 
 static void vdi_aio_cancel(BlockDriverAIOCB *blockacb)
@@ -603,7 +608,7 @@ static void vdi_aio_read_cb(void *opaque, int ret)
     /* prepare next AIO request */
     acb->n_sectors = n_sectors;
     bmap_entry = le32_to_cpu(s->bmap[block_index]);
-    if (bmap_entry == VDI_UNALLOCATED) {
+    if (!VDI_IS_ALLOCATED(bmap_entry)) {
         /* Block not allocated, return zeros, no need to wait. */
         memset(acb->buf, 0, n_sectors * SECTOR_SIZE);
         ret = vdi_schedule_bh(vdi_aio_rw_bh, acb);
@@ -685,7 +690,7 @@ static void vdi_aio_write_cb(void *opaque, int ret)
         if (acb->header_modified) {
             VdiHeader *header = acb->block_buffer;
             logout("now writing modified header\n");
-            assert(acb->bmap_first != VDI_UNALLOCATED);
+            assert(VDI_IS_ALLOCATED(acb->bmap_first));
             *header = s->header;
             vdi_header_to_le(header);
             acb->header_modified = 0;
@@ -699,7 +704,7 @@ static void vdi_aio_write_cb(void *opaque, int ret)
                 goto done;
             }
             return;
-        } else if (acb->bmap_first != VDI_UNALLOCATED) {
+        } else if (VDI_IS_ALLOCATED(acb->bmap_first)) {
             /* One or more new blocks were allocated. */
             uint64_t offset;
             uint32_t bmap_first;
@@ -749,7 +754,7 @@ static void vdi_aio_write_cb(void *opaque, int ret)
     /* prepare next AIO request */
     acb->n_sectors = n_sectors;
     bmap_entry = le32_to_cpu(s->bmap[block_index]);
-    if (bmap_entry == VDI_UNALLOCATED) {
+    if (!VDI_IS_ALLOCATED(bmap_entry)) {
         /* Allocate new block and write to it. */
         uint64_t offset;
         uint8_t *block;
-- 
1.7.6.4