[Qemu-devel] [PATCH v2 0/3] qcow2: Drop REFCOUNT_SHIFT

[Qemu-devel] [PATCH v2 0/3] qcow2: Drop REFCOUNT_SHIFT

[Max Reitz]

In preparation for qemu maybe actually supporting variable refcount
widths in the far future, drop the hardcoded REFCOUNT_SHIFT and instead
use the value as given by the image.

Also, the qcow2 documentation gave the width of a refcount block entry
as sizeof(uint16_t), which is wrong for any refcount order other than 4.
Fix that.

This is a follow-up to my "[PATCH v5 00/11] qcow2: Fix image repairing"
series and therefore depends on it.


v2:
- Patch 2: Fix wrongly fixed calculation [Benoît, Eric]
- Patch 3: Added [Eric]


git-backport-diff against v1:

Key:
[----] : patches are identical
[####] : number of functional differences between upstream/downstream patch
[down] : patch is downstream-only
The flags [FC] indicate (F)unctional and (C)ontextual differences, respectively

001/3:[----] [--] 'qcow2: Drop REFCOUNT_SHIFT'
002/3:[0002] [FC] 'docs/qcow2: Correct refcount_block_entries'
003/3:[down] 'docs/qcow2: Limit refcount_order to [0, 6]'


Max Reitz (3):
  qcow2: Drop REFCOUNT_SHIFT
  docs/qcow2: Correct refcount_block_entries
  docs/qcow2: Limit refcount_order to [0, 6]

 block/qcow2-refcount.c | 32 ++++++++++++++------------------
 block/qcow2.c          |  2 +-
 block/qcow2.h          |  2 --
 docs/specs/qcow2.txt   |  3 ++-
 4 files changed, 17 insertions(+), 22 deletions(-)

-- 
2.1.0

[Qemu-devel] [PATCH v2 1/3] qcow2: Drop REFCOUNT_SHIFT

[Max Reitz]

With BDRVQcowState.refcount_block_bits, we don't need REFCOUNT_SHIFT
anymore.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
---
 block/qcow2-refcount.c | 32 ++++++++++++++------------------
 block/qcow2.c          |  2 +-
 block/qcow2.h          |  2 --
 3 files changed, 15 insertions(+), 21 deletions(-)

diff --git a/block/qcow2-refcount.c b/block/qcow2-refcount.c
index 29136ee..cd6f5a0 100644
--- a/block/qcow2-refcount.c
+++ b/block/qcow2-refcount.c
@@ -102,7 +102,7 @@ static int get_refcount(BlockDriverState *bs, int64_t cluster_index)
     uint16_t *refcount_block;
     uint16_t refcount;
 
-    refcount_table_index = cluster_index >> (s->cluster_bits - REFCOUNT_SHIFT);
+    refcount_table_index = cluster_index >> s->refcount_block_bits;
     if (refcount_table_index >= s->refcount_table_size)
         return 0;
     refcount_block_offset =
@@ -116,8 +116,7 @@ static int get_refcount(BlockDriverState *bs, int64_t cluster_index)
         return ret;
     }
 
-    block_index = cluster_index &
-        ((1 << (s->cluster_bits - REFCOUNT_SHIFT)) - 1);
+    block_index = cluster_index & (s->refcount_block_size - 1);
     refcount = be16_to_cpu(refcount_block[block_index]);
 
     ret = qcow2_cache_put(bs, s->refcount_block_cache,
@@ -152,8 +151,8 @@ static unsigned int next_refcount_table_size(BDRVQcowState *s,
 static int in_same_refcount_block(BDRVQcowState *s, uint64_t offset_a,
     uint64_t offset_b)
 {
-    uint64_t block_a = offset_a >> (2 * s->cluster_bits - REFCOUNT_SHIFT);
-    uint64_t block_b = offset_b >> (2 * s->cluster_bits - REFCOUNT_SHIFT);
+    uint64_t block_a = offset_a >> (s->cluster_bits + s->refcount_block_bits);
+    uint64_t block_b = offset_b >> (s->cluster_bits + s->refcount_block_bits);
 
     return (block_a == block_b);
 }
@@ -174,7 +173,7 @@ static int alloc_refcount_block(BlockDriverState *bs,
     BLKDBG_EVENT(bs->file, BLKDBG_REFBLOCK_ALLOC);
 
     /* Find the refcount block for the given cluster */
-    refcount_table_index = cluster_index >> (s->cluster_bits - REFCOUNT_SHIFT);
+    refcount_table_index = cluster_index >> s->refcount_block_bits;
 
     if (refcount_table_index < s->refcount_table_size) {
 
@@ -243,7 +242,7 @@ static int alloc_refcount_block(BlockDriverState *bs,
 
         /* The block describes itself, need to update the cache */
         int block_index = (new_block >> s->cluster_bits) &
-            ((1 << (s->cluster_bits - REFCOUNT_SHIFT)) - 1);
+            (s->refcount_block_size - 1);
         (*refcount_block)[block_index] = cpu_to_be16(1);
     } else {
         /* Described somewhere else. This can recurse at most twice before we
@@ -315,8 +314,7 @@ static int alloc_refcount_block(BlockDriverState *bs,
     BLKDBG_EVENT(bs->file, BLKDBG_REFTABLE_GROW);
 
     /* Calculate the number of refcount blocks needed so far */
-    uint64_t refcount_block_clusters = 1 << (s->cluster_bits - REFCOUNT_SHIFT);
-    uint64_t blocks_used = DIV_ROUND_UP(cluster_index, refcount_block_clusters);
+    uint64_t blocks_used = DIV_ROUND_UP(cluster_index, s->refcount_block_size);
 
     if (blocks_used > QCOW_MAX_REFTABLE_SIZE / sizeof(uint64_t)) {
         return -EFBIG;
@@ -330,14 +328,14 @@ static int alloc_refcount_block(BlockDriverState *bs,
         uint64_t table_clusters =
             size_to_clusters(s, table_size * sizeof(uint64_t));
         blocks_clusters = 1 +
-            ((table_clusters + refcount_block_clusters - 1)
-            / refcount_block_clusters);
+            ((table_clusters + s->refcount_block_size - 1)
+            / s->refcount_block_size);
         uint64_t meta_clusters = table_clusters + blocks_clusters;
 
         last_table_size = table_size;
         table_size = next_refcount_table_size(s, blocks_used +
-            ((meta_clusters + refcount_block_clusters - 1)
-            / refcount_block_clusters));
+            ((meta_clusters + s->refcount_block_size - 1)
+            / s->refcount_block_size));
 
     } while (last_table_size != table_size);
 
@@ -347,7 +345,7 @@ static int alloc_refcount_block(BlockDriverState *bs,
 #endif
 
     /* Create the new refcount table and blocks */
-    uint64_t meta_offset = (blocks_used * refcount_block_clusters) *
+    uint64_t meta_offset = (blocks_used * s->refcount_block_size) *
         s->cluster_size;
     uint64_t table_offset = meta_offset + blocks_clusters * s->cluster_size;
     uint64_t *new_table = g_try_new0(uint64_t, table_size);
@@ -546,8 +544,7 @@ static int QEMU_WARN_UNUSED_RESULT update_refcount(BlockDriverState *bs,
     {
         int block_index, refcount;
         int64_t cluster_index = cluster_offset >> s->cluster_bits;
-        int64_t table_index =
-            cluster_index >> (s->cluster_bits - REFCOUNT_SHIFT);
+        int64_t table_index = cluster_index >> s->refcount_block_bits;
 
         /* Load the refcount block and allocate it if needed */
         if (table_index != old_table_index) {
@@ -569,8 +566,7 @@ static int QEMU_WARN_UNUSED_RESULT update_refcount(BlockDriverState *bs,
         qcow2_cache_entry_mark_dirty(s->refcount_block_cache, refcount_block);
 
         /* we can update the count and save it */
-        block_index = cluster_index &
-            ((1 << (s->cluster_bits - REFCOUNT_SHIFT)) - 1);
+        block_index = cluster_index & (s->refcount_block_size - 1);
 
         refcount = be16_to_cpu(refcount_block[block_index]);
         refcount += addend;
diff --git a/block/qcow2.c b/block/qcow2.c
index 82bca88..cdbcb81 100644
--- a/block/qcow2.c
+++ b/block/qcow2.c
@@ -1799,7 +1799,7 @@ static int qcow2_create2(const char *filename, int64_t total_size,
         .l1_size                    = cpu_to_be32(0),
         .refcount_table_offset      = cpu_to_be64(cluster_size),
         .refcount_table_clusters    = cpu_to_be32(1),
-        .refcount_order             = cpu_to_be32(3 + REFCOUNT_SHIFT),
+        .refcount_order             = cpu_to_be32(4),
         .header_length              = cpu_to_be32(sizeof(*header)),
     };
 
diff --git a/block/qcow2.h b/block/qcow2.h
index 7c01fb7..5b099cb 100644
--- a/block/qcow2.h
+++ b/block/qcow2.h
@@ -59,8 +59,6 @@
 /* The cluster reads as all zeros */
 #define QCOW_OFLAG_ZERO (1ULL << 0)
 
-#define REFCOUNT_SHIFT 1 /* refcount size is 2 bytes */
-
 #define MIN_CLUSTER_BITS 9
 #define MAX_CLUSTER_BITS 21
 
-- 
2.1.0

[Qemu-devel] [PATCH v2 2/3] docs/qcow2: Correct refcount_block_entries

[Max Reitz]

A refblock entry may have a different size than 16 bits, it may even be
smaller than a byte. Correct the refcount_block_entries calculation
accordingly.

Signed-off-by: Max Reitz <mreitz@redhat.com>
---
 docs/specs/qcow2.txt | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/docs/specs/qcow2.txt b/docs/specs/qcow2.txt
index cfbc8b0..0a878aa 100644
--- a/docs/specs/qcow2.txt
+++ b/docs/specs/qcow2.txt
@@ -183,7 +183,7 @@ blocks and are exactly one cluster in size.
 Given a offset into the image file, the refcount of its cluster can be obtained
 as follows:
 
-    refcount_block_entries = (cluster_size / sizeof(uint16_t))
+    refcount_block_entries = (cluster_size * 8 / refcount_bits)
 
     refcount_block_index = (offset / cluster_size) % refcount_block_entries
     refcount_table_index = (offset / cluster_size) / refcount_block_entries
-- 
2.1.0

Re: [Qemu-devel] [PATCH v2 2/3] docs/qcow2: Correct refcount_block_entries

[Eric Blake]

[-- Attachment #1: Type: text/plain, Size: 1138 bytes --]

On 09/02/2014 04:25 PM, Max Reitz wrote:
> A refblock entry may have a different size than 16 bits, it may even be
> smaller than a byte. Correct the refcount_block_entries calculation
> accordingly.
> 
> Signed-off-by: Max Reitz <mreitz@redhat.com>
> ---
>  docs/specs/qcow2.txt | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)

Reviewed-by: Eric Blake <eblake@redhat.com>

> 
> diff --git a/docs/specs/qcow2.txt b/docs/specs/qcow2.txt
> index cfbc8b0..0a878aa 100644
> --- a/docs/specs/qcow2.txt
> +++ b/docs/specs/qcow2.txt
> @@ -183,7 +183,7 @@ blocks and are exactly one cluster in size.
>  Given a offset into the image file, the refcount of its cluster can be obtained
>  as follows:
>  
> -    refcount_block_entries = (cluster_size / sizeof(uint16_t))
> +    refcount_block_entries = (cluster_size * 8 / refcount_bits)
>  
>      refcount_block_index = (offset / cluster_size) % refcount_block_entries
>      refcount_table_index = (offset / cluster_size) / refcount_block_entries
> 

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org


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Re: [Qemu-devel] [PATCH v2 2/3] docs/qcow2: Correct refcount_block_entries

[Stefan Hajnoczi]

[-- Attachment #1: Type: text/plain, Size: 417 bytes --]

On Wed, Sep 03, 2014 at 12:25:08AM +0200, Max Reitz wrote:
> A refblock entry may have a different size than 16 bits, it may even be
> smaller than a byte. Correct the refcount_block_entries calculation
> accordingly.
> 
> Signed-off-by: Max Reitz <mreitz@redhat.com>
> ---
>  docs/specs/qcow2.txt | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)

Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>

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[Qemu-devel] [PATCH v2 3/3] docs/qcow2: Limit refcount_order to [0, 6]

[Max Reitz]

Specify the upper limit of refcount_order to be 6 (that is,
refcount_bits = 64). Any larger value does not make much sense when all
offsets, sizes, cluster counts etc. "only" have a width of 64 bit as
well, and very large values would be very difficult to support.
Therefore, just cap it at the largest reasonable value.

Suggested-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>
---
 docs/specs/qcow2.txt | 1 +
 1 file changed, 1 insertion(+)

diff --git a/docs/specs/qcow2.txt b/docs/specs/qcow2.txt
index 0a878aa..121dfc8 100644
--- a/docs/specs/qcow2.txt
+++ b/docs/specs/qcow2.txt
@@ -110,6 +110,7 @@ in the description of a field.
                     in bits: refcount_bits = 1 << refcount_order). For version 2
                     images, the order is always assumed to be 4
                     (i.e. refcount_bits = 16).
+                    This value may not exceed 6 (i.e. refcount_bits = 64).
 
         100 - 103:  header_length
                     Length of the header structure in bytes. For version 2
-- 
2.1.0

Re: [Qemu-devel] [PATCH v2 3/3] docs/qcow2: Limit refcount_order to [0, 6]

[Eric Blake]

[-- Attachment #1: Type: text/plain, Size: 1331 bytes --]

On 09/02/2014 04:25 PM, Max Reitz wrote:
> Specify the upper limit of refcount_order to be 6 (that is,
> refcount_bits = 64). Any larger value does not make much sense when all
> offsets, sizes, cluster counts etc. "only" have a width of 64 bit as
> well, and very large values would be very difficult to support.
> Therefore, just cap it at the largest reasonable value.
> 
> Suggested-by: Eric Blake <eblake@redhat.com>
> Signed-off-by: Max Reitz <mreitz@redhat.com>
> ---
>  docs/specs/qcow2.txt | 1 +
>  1 file changed, 1 insertion(+)

Reviewed-by: Eric Blake <eblake@redhat.com>

> 
> diff --git a/docs/specs/qcow2.txt b/docs/specs/qcow2.txt
> index 0a878aa..121dfc8 100644
> --- a/docs/specs/qcow2.txt
> +++ b/docs/specs/qcow2.txt
> @@ -110,6 +110,7 @@ in the description of a field.
>                      in bits: refcount_bits = 1 << refcount_order). For version 2
>                      images, the order is always assumed to be 4
>                      (i.e. refcount_bits = 16).
> +                    This value may not exceed 6 (i.e. refcount_bits = 64).
>  
>          100 - 103:  header_length
>                      Length of the header structure in bytes. For version 2
> 

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org


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Re: [Qemu-devel] [PATCH v2 3/3] docs/qcow2: Limit refcount_order to [0, 6]

[Stefan Hajnoczi]

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On Wed, Sep 03, 2014 at 12:25:09AM +0200, Max Reitz wrote:
> Specify the upper limit of refcount_order to be 6 (that is,
> refcount_bits = 64). Any larger value does not make much sense when all
> offsets, sizes, cluster counts etc. "only" have a width of 64 bit as
> well, and very large values would be very difficult to support.
> Therefore, just cap it at the largest reasonable value.
> 
> Suggested-by: Eric Blake <eblake@redhat.com>
> Signed-off-by: Max Reitz <mreitz@redhat.com>
> ---
>  docs/specs/qcow2.txt | 1 +
>  1 file changed, 1 insertion(+)

Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>

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Re: [Qemu-devel] [PATCH v2 0/3] qcow2: Drop REFCOUNT_SHIFT

[Kevin Wolf]

Am 03.09.2014 um 00:25 hat Max Reitz geschrieben:
> In preparation for qemu maybe actually supporting variable refcount
> widths in the far future, drop the hardcoded REFCOUNT_SHIFT and instead
> use the value as given by the image.
> 
> Also, the qcow2 documentation gave the width of a refcount block entry
> as sizeof(uint16_t), which is wrong for any refcount order other than 4.
> Fix that.
> 
> This is a follow-up to my "[PATCH v5 00/11] qcow2: Fix image repairing"
> series and therefore depends on it.

Reviewed-by: Kevin Wolf <kwolf@redhat.com>

Re: [Qemu-devel] [PATCH v2 0/3] qcow2: Drop REFCOUNT_SHIFT

[Kevin Wolf]

Am 03.09.2014 um 00:25 hat Max Reitz geschrieben:
> In preparation for qemu maybe actually supporting variable refcount
> widths in the far future, drop the hardcoded REFCOUNT_SHIFT and instead
> use the value as given by the image.
> 
> Also, the qcow2 documentation gave the width of a refcount block entry
> as sizeof(uint16_t), which is wrong for any refcount order other than 4.
> Fix that.
> 
> This is a follow-up to my "[PATCH v5 00/11] qcow2: Fix image repairing"
> series and therefore depends on it.

Thanks, applied to the block branch.

Kevin