Re: [Qemu-devel] [PATCH 02/19] block: Add BDS.backing_overridden

[Kevin Wolf]

Am 26.04.2016 um 23:32 hat Max Reitz geschrieben:
> If the backing file is overridden, this most probably does change the
> guest-visible data of a BDS. Therefore, we will need to consider this in
> bdrv_refresh_filename().
> 
> Adding a new field to the BDS is not nice, but it is very simple and
> exactly keeps track of whether the backing file has been overridden.
> 
> Signed-off-by: Max Reitz <address@hidden>

What about live snapshots or block jobs that manipulate bs->backing?
They don't use bdrv_open_backing_file(), but bdrv_append() or
bdrv_replace_in_backing_chain() in order to assign a backing file, which
may or may not be the same as in the image header.

Also, do we want to consider a backing file overridden when the user
specified it explicitly, but it matches the image header?

Kevin

>  block.c                   | 2 ++
>  include/block/block_int.h | 1 +
>  2 files changed, 3 insertions(+)
> 
> diff --git a/block.c b/block.c
> index e349e83..e178488 100644
> --- a/block.c
> +++ b/block.c
> @@ -1299,6 +1299,7 @@ int bdrv_open_backing_file(BlockDriverState *bs, QDict 
> *parent_options,
>  
>      reference = qdict_get_try_str(parent_options, bdref_key);
>      if (reference || qdict_haskey(options, "file.filename")) {
> +        bs->backing_overridden = true;
>          backing_filename[0] = '\0';
>      } else if (bs->backing_file[0] == '\0' && qdict_size(options) == 0) {
>          QDECREF(options);
> @@ -1589,6 +1590,7 @@ static int bdrv_open_inherit(BlockDriverState **pbs, 
> const char *filename,
>      backing = qdict_get_try_str(options, "backing");
>      if (backing && *backing == '\0') {
>          flags |= BDRV_O_NO_BACKING;
> +        bs->backing_overridden = true;
>          qdict_del(options, "backing");
>      }
>  
> diff --git a/include/block/block_int.h b/include/block/block_int.h
> index 10d8759..d73e9ce 100644
> --- a/include/block/block_int.h
> +++ b/include/block/block_int.h
> @@ -409,6 +409,7 @@ struct BlockDriverState {
>      char backing_file[PATH_MAX]; /* if non zero, the image is a diff of
>                                      this file image */
>      char backing_format[16]; /* if non-zero and backing_file exists */
> +    bool backing_overridden; /* backing file has been specified by the user 
> */
>  
>      QDict *full_open_options;
>      char exact_filename[PATH_MAX];
> -- 
> 2.8.0
>