Re: [Qemu-devel] [PATCH v13 05/14] block: Add bdrv_set_backing_hd()

["Benoît Canet" <benoit.canet@irqsave.net>]

The Wednesday 29 Jan 2014 à 13:07:32 (+0800), Fam Zheng wrote :
> This is the common but non-trivial steps to assign or change the
> backing_hd of BDS.
> 
> Signed-off-by: Fam Zheng <famz@redhat.com>
> ---
>  block.c               | 34 +++++++++++++++++++++++++---------
>  include/block/block.h |  1 +
>  2 files changed, 26 insertions(+), 9 deletions(-)
> 
> diff --git a/block.c b/block.c
> index 8000ac0..c4eaa37 100644
> --- a/block.c
> +++ b/block.c
> @@ -1072,6 +1072,26 @@ fail:
>      return ret;
>  }
>  
> +void bdrv_set_backing_hd(BlockDriverState *bs, BlockDriverState *backing_hd)
> +{
> +    if (bs->backing_hd) {
> +        bdrv_unref(bs->backing_hd);
> +    }
> +
> +    bs->backing_hd = backing_hd;
> +    if (!backing_hd) {
> +        bs->backing_file[0] = '\0';
> +        bs->backing_format[0] = '\0';
> +        goto out;
> +    }
> +    pstrcpy(bs->backing_file, sizeof(bs->backing_file), backing_hd->filename);


> +    pstrcpy(bs->backing_format, sizeof(bs->backing_file),

I don't understand the sizeof(bs->backing_file). Is it sizeof(bs->backing_format) ?

> +            backing_hd->drv ? backing_hd->drv->format_name : "");
> +    bdrv_ref(bs->backing_hd);
> +out:
> +    bdrv_refresh_limits(bs);
> +}
> +
>  /*
>   * Opens the backing file for a BlockDriverState if not yet open
>   *
> @@ -1085,6 +1105,7 @@ int bdrv_open_backing_file(BlockDriverState *bs, QDict *options, Error **errp)
>      char backing_filename[PATH_MAX];
>      int back_flags, ret;
>      BlockDriver *back_drv = NULL;
> +    BlockDriverState *backing_hd;
>      Error *local_err = NULL;
>  
>      if (bs->backing_hd != NULL) {
> @@ -1108,7 +1129,7 @@ int bdrv_open_backing_file(BlockDriverState *bs, QDict *options, Error **errp)
>                                         sizeof(backing_filename));
>      }
>  
> -    bs->backing_hd = bdrv_new("");
> +    backing_hd = bdrv_new("");
>  
>      if (bs->backing_format[0] != '\0') {
>          back_drv = bdrv_find_format(bs->backing_format);
> @@ -1118,23 +1139,18 @@ int bdrv_open_backing_file(BlockDriverState *bs, QDict *options, Error **errp)
>      back_flags = bs->open_flags & ~(BDRV_O_RDWR | BDRV_O_SNAPSHOT |
>                                      BDRV_O_COPY_ON_READ);
>  
> -    ret = bdrv_open(bs->backing_hd,
> +    ret = bdrv_open(backing_hd,
>                      *backing_filename ? backing_filename : NULL, options,
>                      back_flags, back_drv, &local_err);
>      if (ret < 0) {
> -        bdrv_unref(bs->backing_hd);
> -        bs->backing_hd = NULL;
> +        bdrv_unref(backing_hd);
Here I wonder if this way of calling bdrv_open and doing a bdrv_unref confict
with Max latests bdrv_open patches.
You probably need to rebase and get rid of this bdrv_unref() : check Max series
to be sure.

>          bs->open_flags |= BDRV_O_NO_BACKING;
>          error_setg(errp, "Could not open backing file: %s",
>                     error_get_pretty(local_err));
>          error_free(local_err);
>          return ret;
>      }
> -
> -    if (bs->backing_hd->file) {
> -        pstrcpy(bs->backing_file, sizeof(bs->backing_file),
> -                bs->backing_hd->file->filename);
> -    }
> +    bdrv_set_backing_hd(bs, backing_hd);
>  
>      /* Recalculate the BlockLimits with the backing file */
>      bdrv_refresh_limits(bs);
> diff --git a/include/block/block.h b/include/block/block.h
> index 9125bbe..f449753 100644
> --- a/include/block/block.h
> +++ b/include/block/block.h
> @@ -208,6 +208,7 @@ int bdrv_file_open(BlockDriverState **pbs, const char *filename,
>  int bdrv_open_image(BlockDriverState **pbs, const char *filename,
>                      QDict *options, const char *bdref_key, int flags,
>                      bool force_raw, bool allow_none, Error **errp);
> +void bdrv_set_backing_hd(BlockDriverState *bs, BlockDriverState *backing_hd);
>  int bdrv_open_backing_file(BlockDriverState *bs, QDict *options, Error **errp);
>  int bdrv_open(BlockDriverState *bs, const char *filename, QDict *options,
>                int flags, BlockDriver *drv, Error **errp);
> -- 
> 1.8.5.3
> 
> 

bdrv_set_backing_hd seems to be handy since some people want to dynamically
change backing_file using QMP to move them from slow storage to fast storage and
the reverse.

Best regards

Benoît