Re: [PATCH 09/10] migration/colo: Move qemu_fflush() closer to its user for fb

[Fabiano Rosas <farosas@suse.de>]

Peter Xu <peterx@redhat.com> writes:

> COLO caches all device states in a buffer channel `fb'.  Flush it right
> before pushing the buffer to make sure everything needed will be flushed,
> add a comment explaning it.
>
> Signed-off-by: Peter Xu <peterx@redhat.com>
> ---
>  migration/colo.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/migration/colo.c b/migration/colo.c
> index f92803dd29..e2307f1907 100644
> --- a/migration/colo.c
> +++ b/migration/colo.c
> @@ -472,8 +472,6 @@ static int colo_do_checkpoint_transaction(MigrationState *s,
>      qemu_savevm_state_complete_precopy_iterable(s->to_dst_file, false);
>      qemu_put_byte(s->to_dst_file, QEMU_VM_EOF);
>  
> -    qemu_fflush(fb);
> -
>      /*
>       * We need the size of the VMstate data in Secondary side,
>       * With which we can decide how much data should be read.
> @@ -484,6 +482,8 @@ static int colo_do_checkpoint_transaction(MigrationState *s,
>          goto out;
>      }
>  
> +    /* Flush to make sure everything lands bioc->data */
> +    qemu_fflush(fb);
>      qemu_put_buffer(s->to_dst_file, bioc->data, bioc->usage);
>      ret = qemu_fflush(s->to_dst_file);

Reviewed-by: Fabiano Rosas <farosas@suse.de>

Wait, aren't we wasting a memcpy here? Isn't this the same, but without
the copy of bioc->data to s->to_dst_file->buf:

qemu_fflush(fb);                                        // ensure bioc->data is populated
qemu_fflush(s->to_dst_file);                            // send buffered data from migration stream
qemu_put_buffer_async(s->to_dst_file, bioc->data, ...); // point s->to_dst_file->iov to bioc->data
qemu_fflush(s->to_dst_file);                            // send s->to_dst_file->iov contents

>      if (ret < 0) {