Re: [Qemu-devel] [PATCH v4 1/4] block: pass bdrv_* methods to bs->file by default in block filters

[Eric Blake]

On 07/11/2017 11:37 AM, Manos Pitsidianakis wrote:
> The following functions fail if bs->drv is a filter and does not
> implement them:
> 
> bdrv_probe_blocksizes
> bdrv_probe_geometry
> bdrv_truncate
> bdrv_has_zero_init
> bdrv_get_info
> 
> Instead, the call should be passed to bs->file if it exists, to allow
> filter drivers to support those methods without implementing them. This
> commit makes `drv->is_filter = true` imply that these callbacks will be
> forwarded to bs->file by default, so disabling support for these
> functions must be done explicitly.
> 
> Signed-off-by: Manos Pitsidianakis <address@hidden>
> ---
>  block.c                   | 21 +++++++++++++++++++--
>  include/block/block_int.h |  6 +++++-
>  2 files changed, 24 insertions(+), 3 deletions(-)

> @@ -3406,11 +3410,15 @@ int bdrv_truncate(BdrvChild *child, int64_t offset, 
> Error **errp)
>  
>      assert(child->perm & BLK_PERM_RESIZE);
>  
> +    /* if bs->drv == NULL, bs is closed, so there's nothing to do here */
>      if (!drv) {
>          error_setg(errp, "No medium inserted");
>          return -ENOMEDIUM;
>      }
>      if (!drv->bdrv_truncate) {
> +        if (bs->file && drv->is_filter) {
> +            return bdrv_truncate(bs->file, offset, errp);
> +        }

This has a semantic (but not merge) conflict with Max's preallocation
work, and it started to be non-trivial for me to adjust locally in my
testing. It may be easiest if you send a v5 on top of
'https://github.com/XanClic/qemu.git block', since Max already has a
pending pull request:

https://lists.gnu.org/archive/html/qemu-devel/2017-07/msg02841.html

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org

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