Re: [PATCH v5 13/31] qcow2: Update get/set_l2_entry() and add get/set_l2_bitmap()

[Eric Blake <eblake@redhat.com>]

On 5/5/20 12:38 PM, Alberto Garcia wrote:
> Extended L2 entries are 128-bit wide: 64 bits for the entry itself and
> 64 bits for the subcluster allocation bitmap.
> 
> In order to support them correctly get/set_l2_entry() need to be
> updated so they take the entry width into account in order to
> calculate the correct offset.
> 
> This patch also adds the get/set_l2_bitmap() functions that are
> used to access the bitmaps. For convenience we allow calling
> get_l2_bitmap() on images without subclusters. In this case the
> returned value is always 0 and has no meaning.
> 
> Signed-off-by: Alberto Garcia <berto@igalia.com>
> ---
>   block/qcow2.h | 21 +++++++++++++++++++++
>   1 file changed, 21 insertions(+)
> 

> +static inline void set_l2_bitmap(BDRVQcow2State *s, uint64_t *l2_slice,
> +                                 int idx, uint64_t bitmap)
> +{
> +    assert(has_subclusters(s));
> +    idx *= l2_entry_size(s) / sizeof(uint64_t);
> +    l2_slice[idx + 1] = cpu_to_be64(bitmap);
> +}

Unrelated to this patch, but I just thought of it:

What happens for an image whose size is not cluster-aligned?  Must the 
bits corresponding to subclusters not present in the final cluster 
always be zero, or are they instead ignored regardless of value?

But for this patch:

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

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