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Re: [PATCH v5 07/31] qcow2: Document the Extended L2 Entries feature


From: Eric Blake
Subject: Re: [PATCH v5 07/31] qcow2: Document the Extended L2 Entries feature
Date: Tue, 5 May 2020 14:35:06 -0500
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.7.0

On 5/5/20 12:38 PM, Alberto Garcia wrote:
Subcluster allocation in qcow2 is implemented by extending the
existing L2 table entries and adding additional information to
indicate the allocation status of each subcluster.

This patch documents the changes to the qcow2 format and how they
affect the calculation of the L2 cache size.

Signed-off-by: Alberto Garcia <address@hidden>
Reviewed-by: Max Reitz <address@hidden>
---
  docs/interop/qcow2.txt | 68 ++++++++++++++++++++++++++++++++++++++++--
  docs/qcow2-cache.txt   | 19 +++++++++++-
  2 files changed, 83 insertions(+), 4 deletions(-)


@@ -547,7 +557,8 @@ Standard Cluster Descriptor:
                      nor is data read from the backing file if the cluster is
                      unallocated.
- With version 2, this is always 0.
+                    With version 2 or with extended L2 entries (see the next
+                    section), this is always 0.

In your cover letter, you said you changed things to tolerate this bit being set even with extended L2 entries. Does this sentence need a tweak?

1 - 8: Reserved (set to 0) @@ -584,6 +595,57 @@ file (except if bit 0 in the Standard Cluster Descriptor is set). If there is
  no backing file or the backing file is smaller than the image, they shall read
  zeros for all parts that are not covered by the backing file.
+== Extended L2 Entries ==
+
+An image uses Extended L2 Entries if bit 4 is set on the incompatible_features
+field of the header.
+
+In these images standard data clusters are divided into 32 subclusters of the
+same size. They are contiguous and start from the beginning of the cluster.
+Subclusters can be allocated independently and the L2 entry contains 
information
+indicating the status of each one of them. Compressed data clusters don't have
+subclusters so they are treated the same as in images without this feature.
+
+The size of an extended L2 entry is 128 bits so the number of entries per table
+is calculated using this formula:
+
+    l2_entries = (cluster_size / (2 * sizeof(uint64_t)))
+
+The first 64 bits have the same format as the standard L2 table entry described
+in the previous section, with the exception of bit 0 of the standard cluster
+descriptor.

Also this sentence.

+
+The last 64 bits contain a subcluster allocation bitmap with this format:
+
+Subcluster Allocation Bitmap (for standard clusters):
+
+    Bit  0 -  31:   Allocation status (one bit per subcluster)

Why two spaces after '-'? I understand it in situations like '0 - 3' in the same list as '16 - 19', to make for a right-justified column, but here, everything in the second column is two digits, so the extra padding doesn't add anything useful. Or did you mean to have '64 - 95' and '96 - 127', making it obvious that these are the second set of bits on top of the existing bits in the first 8 bytes?

+
+                    1: the subcluster is allocated. In this case the
+                       host cluster offset field must contain a valid
+                       offset.
+                    0: the subcluster is not allocated. In this case
+                       read requests shall go to the backing file or
+                       return zeros if there is no backing file data.
+
+                    Bits are assigned starting from the least significant
+                    one (i.e. bit x is used for subcluster x).
+
+        32 -  63    Subcluster reads as zeros (one bit per subcluster)
+
+                    1: the subcluster reads as zeros. In this case the
+                       allocation status bit must be unset. The host
+                       cluster offset field may or may not be set.
+                    0: no effect.
+
+                    Bits are assigned starting from the least significant
+                    one (i.e. bit x is used for subcluster x - 32).

Of course, if you change to 64-95 and 96-127, the two sentences mapping bit x to subcluster y need adjusting by 64 as well.

+
+Subcluster Allocation Bitmap (for compressed clusters):
+
+    Bit  0 -  63:   Reserved (set to 0)
+                    Compressed clusters don't have subclusters,
+                    so this field is not used.

I can live with the wording as-is (since you did call out the "second 64 bits" or with the adjusted bit numberings.

--
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org




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