Re: [Qemu-devel] [PATCH v2] spec/qcow2: bitmaps: zero bitmap table offset

[Kevin Wolf <kwolf@redhat.com>]

Am 30.06.2016 um 19:23 hat Denis V. Lunev geschrieben:
> On 06/30/2016 07:40 PM, John Snow wrote:
> >
> >On 06/30/2016 05:12 AM, Denis V. Lunev wrote:
> >>On 06/30/2016 10:34 AM, Vladimir Sementsov-Ogievskiy wrote:
> >>>After loading bitmap from image and setting IN_USE flag in it's header,
> >>>corresponding data (bitmap table and data clusters) becomes inconsistent
> >>>and is no longer needed. It is better to free bitmap table and
> >>>corresponding clusters from the image immediately after loading the
> >>>bitmap than free them when the bitmap is saved, or deleted or set
> >>>non-persistent.
> >>>
> >>>For now it is impossible to store only bitmap header without bitmap
> >>>table, as specification requires it. Storing zeroed bitmap table (one or
> >>>more clusters) is the only option to implement the behaviour similar to
> >>>specified above.
> >>>
> >>>The same problem is for just storing empty bitmaps.
> >>>
> >>>This patch allows storing only bitmap header for empty bitmaps.
> >>>
> >>>Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
> >>>---
> >>>
> >>>Additional note. Should we also allow here bitmap_table_offset = 1, like
> >>>in bitmap table, for the bitmap with all bits set? I am not sure that it
> >>>is needed at all, but just to keep the company..
> >>>
> >>>   docs/specs/qcow2.txt | 5 ++++-
> >>>   1 file changed, 4 insertions(+), 1 deletion(-)
> >>>
> >>>diff --git a/docs/specs/qcow2.txt b/docs/specs/qcow2.txt
> >>>index 80cdfd0..9826222 100644
> >>>--- a/docs/specs/qcow2.txt
> >>>+++ b/docs/specs/qcow2.txt
> >>>@@ -435,9 +435,12 @@ Structure of a bitmap directory entry:
> >>>                       Offset into the image file at which the bitmap
> >>>table
> >>>                       (described below) for the bitmap starts. Must be
> >>>aligned to
> >>>                       a cluster boundary.
> >>>+                    Zero value means that bitmap table is not
> >>>allocated and the
> >>>+                    bitmap should be considered as empty (all bits
> >>>are zero).
> >>>              8 - 11:    bitmap_table_size
> >>>-                    Number of entries in the bitmap table of the bitmap.
> >>>+                    Number of entries in the bitmap table of the
> >>>bitmap. It
> >>>+                    must be zero if bitmap_table_offset is zero.
> >>>             12 - 15:    flags
> >>>                       Bit
> >>NACK
> >>
> >>no guys, we can not make this change at the moment.
> >>We do have QEMU available in the field which is working
> >>with the currently specified format.
> >>
> >>Den
> >>
> >But I think the new format is a /compatible/ change. Under the old spec,
> >I think this field is *NEVER* zero.
> >
> >Am I wrong?
> yes
> 
> but as far as I can understand this breaks backward compatibility,
> i.e. software working with the current revision of the specification
> will not be able to load new images.

This is okay if it means that it cleanly errors out. I'm a bit worried
that it might take the 0 literally, though, and use the image header as
the bitmap table. Can you check that, Den?

If that's the case, we can make the change only with an incompatible
feature bit, and that's probably not worth it.

Kevin