Re: [Qemu-devel] [PATCH v3 2/8] block: set snapshot option for block devices in blkreplay module

["Pavel Dovgalyuk" <dovgaluk@ispras.ru>]

> From: Kevin Wolf [mailto:kwolf@redhat.com]
> Am 20.09.2016 um 14:31 hat Pavel Dovgalyuk geschrieben:
> > This patch adds overlay option for blkreplay filter.
> > It allows creating persistent overlay file for saving and reloading
> > VM snapshots in record/replay modes.
> >
> > Signed-off-by: Pavel Dovgalyuk <pavel.dovgaluk@ispras.ru>
> > ---
> >  block/blkreplay.c |  119 +++++++++++++++++++++++++++++++++++++++++++++++++++++
> >  docs/replay.txt   |    8 ++++
> >  vl.c              |    2 -
> >  3 files changed, 128 insertions(+), 1 deletion(-)
> >
> > diff --git a/block/blkreplay.c b/block/blkreplay.c
> > index 30f9d5f..e90d2c6 100644
> > --- a/block/blkreplay.c
> > +++ b/block/blkreplay.c
> > @@ -14,6 +14,7 @@
> >  #include "block/block_int.h"
> >  #include "sysemu/replay.h"
> >  #include "qapi/error.h"
> > +#include "qapi/qmp/qstring.h"
> >
> >  typedef struct Request {
> >      Coroutine *co;
> > @@ -25,11 +26,82 @@ typedef struct Request {
> >     block devices should not get overlapping ids. */
> >  static uint64_t request_id;
> >
> > +static BlockDriverState *blkreplay_append_snapshot(BlockDriverState *bs,
> > +                                                   int flags,
> > +                                                   QDict *snapshot_options,
> > +                                                   Error **errp)
> > +{
> > +    int ret;
> > +    BlockDriverState *bs_snapshot;
> > +
> > +    /* Create temporary file, if needed */
> > +    if ((flags & BDRV_O_TEMPORARY) || replay_mode == REPLAY_MODE_RECORD) {
> > +        int64_t total_size;
> > +        QemuOpts *opts = NULL;
> > +        const char *tmp_filename = qdict_get_str(snapshot_options,
> > +                                                 "file.filename");
> > +
> > +        /* Get the required size from the image */
> > +        total_size = bdrv_getlength(bs);
> > +        if (total_size < 0) {
> > +            error_setg_errno(errp, -total_size, "Could not get image size");
> > +            goto out;
> > +        }
> > +
> > +        opts = qemu_opts_create(bdrv_qcow2.create_opts, NULL, 0,
> > +                                &error_abort);
> > +        qemu_opt_set_number(opts, BLOCK_OPT_SIZE, total_size, &error_abort);
> > +        ret = bdrv_create(&bdrv_qcow2, tmp_filename, opts, errp);
> > +        qemu_opts_del(opts);
> > +        if (ret < 0) {
> > +            error_prepend(errp, "Could not create temporary overlay '%s': ",
> > +                          tmp_filename);
> > +            goto out;
> > +        }
> > +    }
> > +
> > +    bs_snapshot = bdrv_open(NULL, NULL, snapshot_options, flags, errp);
> > +    snapshot_options = NULL;
> > +    if (!bs_snapshot) {
> > +        ret = -EINVAL;
> > +        goto out;
> > +    }
> > +
> > +    /* bdrv_append() consumes a strong reference to bs_snapshot (i.e. it will
> > +     * call bdrv_unref() on it), so in order to be able to return one, we have
> > +     * to increase bs_snapshot's refcount here */
> > +    bdrv_ref(bs_snapshot);
> > +    bdrv_append(bs_snapshot, bs);
> > +
> > +    return bs_snapshot;
> > +
> > +out:
> > +    QDECREF(snapshot_options);
> > +    return NULL;
> > +}
> > +
> > +static QemuOptsList blkreplay_runtime_opts = {
> > +    .name = "blkreplay",
> > +    .head = QTAILQ_HEAD_INITIALIZER(blkreplay_runtime_opts.head),
> > +    .desc = {
> > +        {
> > +            .name = "overlay",
> > +            .type = QEMU_OPT_STRING,
> > +            .help = "Optional overlay file for snapshots",
> > +        },
> > +        { /* end of list */ }
> > +    },
> > +};
> > +
> >  static int blkreplay_open(BlockDriverState *bs, QDict *options, int flags,
> >                            Error **errp)
> >  {
> >      Error *local_err = NULL;
> >      int ret;
> > +    QDict *snapshot_options = qdict_new();
> > +    int snapshot_flags = BDRV_O_RDWR;
> > +    const char *snapshot;
> > +    QemuOpts *opts = NULL;
> >
> >      /* Open the image file */
> >      bs->file = bdrv_open_child(NULL, options, "image",
> > @@ -40,6 +112,43 @@ static int blkreplay_open(BlockDriverState *bs, QDict *options, int
> flags,
> >          goto fail;
> >      }
> >
> > +    opts = qemu_opts_create(&blkreplay_runtime_opts, NULL, 0, &error_abort);
> > +    qemu_opts_absorb_qdict(opts, options, &local_err);
> > +    if (local_err) {
> > +        ret = -EINVAL;
> > +        goto fail;
> > +    }
> 
> Starting from here...
> 
> > +    /* Prepare options QDict for the overlay file */
> > +    qdict_put(snapshot_options, "file.driver",
> > +              qstring_from_str("file"));
> > +    qdict_put(snapshot_options, "driver",
> > +              qstring_from_str("qcow2"));
> > +
> > +    snapshot = qemu_opt_get(opts, "overlay");
> > +    if (snapshot) {
> > +        qdict_put(snapshot_options, "file.filename",
> > +                  qstring_from_str(snapshot));
> > +    } else {
> > +        char tmp_filename[PATH_MAX + 1];
> > +        ret = get_tmp_filename(tmp_filename, PATH_MAX + 1);
> > +        if (ret < 0) {
> > +            error_setg_errno(errp, -ret, "Could not get temporary filename");
> > +            goto fail;
> > +        }
> > +        qdict_put(snapshot_options, "file.filename",
> > +                  qstring_from_str(tmp_filename));
> > +        snapshot_flags |= BDRV_O_TEMPORARY;
> > +    }
> > +
> > +    /* Add temporary snapshot to preserve the image */
> > +    if (!blkreplay_append_snapshot(bs->file->bs, snapshot_flags,
> > +                      snapshot_options, &local_err)) {
> > +        ret = -EINVAL;
> > +        error_propagate(errp, local_err);
> > +        goto fail;
> > +    }
> 
> ...to here, this is code that is written according to a fundamentally
> wrong design.
> 
> The problem here is that you're hard-coding the options that are used
> for the overlay image. This restricts users to using only qcow2 (there
> are many other formats that support backing files), only on local files
> (there are quite a few more protocols over which qcow2 works), only with
> default options (e.g. cache mode, discard behaviour, lazy refcounts).
> 
> The correct way would be to get not a filename option, but what is
> called a BlockdevRef in QAPI: Either a node-name of a separately created
> BDS or an inline definition of a new BDS. Have a look at other filter
> drivers for how to do this. The thing to look for is bdrv_open_child()
> (though of course an overlay may look a bit different from this).

Thanks.
Then I'll better apply Paolo's approach to creating overlays.

Pavel Dovgalyuk