Re: [PATCH 4/6] block, migration: add bdrv_finalize_vmstate helper

[Daniel P . Berrangé]

On Thu, Jul 09, 2020 at 04:26:42PM +0300, Denis V. Lunev wrote:
> Right now bdrv_fclose() is just calling bdrv_flush().
> 
> The problem is that migration code is working inefficiently from block
> layer terms and are frequently called for very small pieces of
> unaligned data. Block layer is capable to work this way, but this is very
> slow.
> 
> This patch is a preparation for the introduction of the intermediate
> buffer at block driver state. It would be beneficial to separate
> conventional bdrv_flush() from closing QEMU file from migration code.
> 
> The patch also forces bdrv_finalize_vmstate() operation inside
> synchronous blk_save_vmstate() operation. This helper is used from
> qemu-io only.
> 
> Signed-off-by: Denis V. Lunev <den@openvz.org>
> Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
> CC: Kevin Wolf <kwolf@redhat.com>
> CC: Max Reitz <mreitz@redhat.com>
> CC: Stefan Hajnoczi <stefanha@redhat.com>
> CC: Fam Zheng <fam@euphon.net>
> CC: Juan Quintela <quintela@redhat.com>
> CC: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
> CC: Denis Plotnikov <dplotnikov@virtuozzo.com>
> ---
>  block/block-backend.c |  6 +++++-
>  block/io.c            | 15 +++++++++++++++
>  include/block/block.h |  5 +++++
>  migration/savevm.c    |  4 ++++
>  4 files changed, 29 insertions(+), 1 deletion(-)

> diff --git a/migration/savevm.c b/migration/savevm.c
> index 45c9dd9d8a..d8a94e312c 100644
> --- a/migration/savevm.c
> +++ b/migration/savevm.c
> @@ -150,6 +150,10 @@ static ssize_t block_get_buffer(void *opaque, uint8_t 
> *buf, int64_t pos,
>  
>  static int bdrv_fclose(void *opaque, Error **errp)
>  {
> +    int err = bdrv_finalize_vmstate(opaque);
> +    if (err < 0) {
> +        return err;

This is returning an error without having populating 'errp' which means
the caller will be missing error diagnosis

> +    }
>      return bdrv_flush(opaque);
>  }

Regards,
Daniel
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