Re: [Qemu-devel] [PATCH v9 04/31] block: Add BDS.auto_backing_file

[Eric Blake <eblake@redhat.com>]

On 06/27/2018 07:07 PM, Max Reitz wrote:
> 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().
> 

> So all in all, there will be false negatives where (as of a future
> patch) bdrv_refresh_filename() will assume that the backing file differs
> from what was specified in the image header, even though it really does
> not.  However, these cases should be limited to where (1) the user
> actually did override something in the backing chain (e.g. by specifying
> options for the backing file), or (2) the user executed a QMP command to
> change some node's backing file (e.g. change-backing-file or
> block-commit with @backing-file given) where the given filename does not
> happen to coincide with qemu's idea of the backing BDS's filename.
> 
> Then again, (1) really is limited to -drive.  With -blockdev or
> blockdev-add, you have to adhere to the schema, so a user cannot give
> partial "unimportant" options (e.g. by just setting backing.node-name
> and leaving the rest to the image header).  Therefore, trying to fix
> this would mean trying to fix something for -drive only.
> 
> To improve on (2), we would need a full infrastructure to "canonicalize"
> an arbitrary filename (+ options), so it can be compared against
> another.  That seems a bit over the top, considering that filenames
> nowadays are there mostly for the user's entertainment.

Lengthy commit message, but the rationale is sound and useful.

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

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
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