On Tue, May 27, 2014 at 02:30:02PM -0600, Eric Blake wrote:
> On 05/27/2014 08:28 AM, Jeff Cody wrote:
> > On some image chains, QEMU may not always be able to resolve the
> > filenames properly, when updating the backing file of an image
> > after a block job.
> >
> > For instance, certain relative pathnames may fail, or drives may
> > have been specified originally by file descriptor (e.g. /dev/fd/???),
> > or a relative protocol pathname may have been used.
> >
> > In these instances, QEMU may lack the information to be able to make
> > the correct choice, but the user or management layer most likely does
> > have that knowledge.
> >
> > With this extension to the block-stream api, the user is able to change
> > the backing file of the active layer as part of the block-stream
> > operation.
> >
> > This allows the change to be 'safe', in the sense that if the attempt
> > to write the active image metadata fails, then the block-stream
> > operation returns failure, without disrupting the guest.
> >
> > If a backing file string is not specified in the command, the backing
> > file string to use is determined in the same manner as it was
> > previously.
> >
> > Signed-off-by: Jeff Cody <jc...@redhat.com>
> > ---
> > block/stream.c | 13 ++++++++-----
> > blockdev.c | 12 +++++++++++-
> > hmp-commands.hx | 2 +-
> > hmp.c | 2 ++
> > include/block/block_int.h | 2 +-
> > qapi-schema.json | 18 +++++++++++++++++-
> > qmp-commands.hx | 2 +-
> > 7 files changed, 41 insertions(+), 10 deletions(-)
> >
>
> > @@ -220,7 +221,7 @@ void stream_start(BlockDriverState *bs,
> > BlockDriverState *base,
> > const char *base_id, int64_t speed,
> > BlockdevOnError on_error,
> > BlockDriverCompletionFunc *cb,
> > - void *opaque, Error **errp)
> > + void *opaque, const char *backing_file_str, Error **errp)
> > {
>
> Umm, aren't the 'base_id' and 'backing_file_str' arguments redundant
> now? You only need one, not both, because there is only one backing
> string to be written (or cleared) into the active file. [1]
>
Yes, we can get rid of the extra argument... I had originally done
things differently in stream.c (used the optional backing file string
later at the end of the block job). When I realized the string wasn't
used anywhere else, I moved it into the stream_start().... I should
have factored out the extra parameter at that point, too.
I'll remove it for v3.
> > @@ -237,8 +238,10 @@ void stream_start(BlockDriverState *bs,
> > BlockDriverState *base,
> > }
> >
> > s->base = base;
> > - if (base_id) {
> > - pstrcpy(s->backing_file_id, sizeof(s->backing_file_id), base_id);
> > + if (backing_file_str) {
> > + s->backing_file_str = g_strdup(backing_file_str);
> > + } else {
> > + s->backing_file_str = g_strdup(base_id);
>
> g_strdup(NULL) is safely NULL, correct? (You hit this case when base_id
> is NULL, which happens when base is NULL).
>
Yes, g_strdup(NULL) returns NULL, so I relied on that.
Once we remove the extra argument, this change will become:
- if (base_id) {
- pstrcpy(s->backing_file_id, sizeof(s->backing_file_id), base_id);
- }
+ s->backing_file_str = g_strdup(base_id);
> > @@ -1941,8 +1942,17 @@ void qmp_block_stream(bool has_device, const char
> > *device,
> > return;
> > }
> >
> > + /* if we are streaming from the bottommost base image, then specifying
> > + * a backing file does not make sense, and is an error */
>
> Misleading (back to the complaint in 9/11) - omitting base is different
> from using the bottommost base image as base. I'd word that more like:
>
> If we are streaming the entire chain, then the result will have no
> backing file and specifying a backing name is an error
>
Agree
> > + if (base_bs == NULL && has_backing_file) {
> > + error_setg(errp, "backing file specified, but streaming from "
> > + "bottommost base image");
> > + return;
> > + }
>
> The 'if' condition is correct and necessary, but the error message could
> use better wording; maybe:
>
> backing file specified but no base image supplied
>
Yes, the error message isn't great - I'll use something like yours.
> > stream_start(bs, base_bs, base_name, has_speed ? speed : 0,
> > - on_error, block_job_cb, bs, &local_err);
> > + on_error, block_job_cb, bs,
> > + has_backing_file ? backing_file : NULL, &local_err);
>
> [1] Again, how do 'base_name' and 'backing_file' differ at this point in
> the game? Isn't it just simpler to do:
> has_backing_file ? backing_file : base_name
> and use a single parameter?
>
> > +++ b/qapi-schema.json
> > @@ -2611,6 +2611,21 @@
> > # @base-node-name: #optional the block driver state node name of the
> > # common backing file. (Since 2.1)
> > #
> > +# @backing-file: #optional The backing file string to write into the active
> > +# layer. This filename is not validated.
> > +#
> > +# If a pathname string is such that it cannot be
> > +# resolved be QEMU, that means that subsequent
> > QMP or
>
> Copy-and-paste strikes again :)
> s/be/by/
Thanks. The funny thing is I read that sentence a few times before I
noticed it, even after you pointed it out.
>
> > +++ b/qmp-commands.hx
> > @@ -979,7 +979,7 @@ EQMP
> >
> > {
> > .name = "block-stream",
> > - .args_type =
> > "device:B?,base:s?,base-node-name:s?,speed:o?,on-error:s?",
> > + .args_type =
> > "device:B?,base:s?,base-node-name:s?,speed:o?,backing-file:s?,on-error:s?",
> > .mhandler.cmd_new = qmp_marshal_input_block_stream,
> > },
>
> Missing documentation of backing-file? Oh, the entire block-stream call
> is missing documentation, when compared to the block-commit call. Oh
> well, I can't fault you for that, although it might be nice to rectify
> while we are improving it.
>
I actually thought about adding it in, but got lazy :). Since I'll be
putting out a v3, I'll go ahead and add documentation for
block-stream.
