Re: [Xen-devel] [PATCH v10 10/31] tools/libxl: add back channel support to write stream

[Wei Liu <wei.liu2@xxxxxxxxxx>]

On Mon, Feb 22, 2016 at 10:52:14AM +0800, Wen Congyang wrote:
> Add back channel support to write stream. If the write stream is
> a back channel stream, this means the write stream is used by
> Secondary to send some records back.
> 
> Signed-off-by: Yang Hongyang <hongyang.yang@xxxxxxxxxxxx>
> Signed-off-by: Wen Congyang <wency@xxxxxxxxxxxxxx>
> ---
>  tools/libxl/libxl_dom_save.c     |  1 +
>  tools/libxl/libxl_internal.h     |  1 +
>  tools/libxl/libxl_stream_write.c | 26 ++++++++++++++++++++------
>  3 files changed, 22 insertions(+), 6 deletions(-)
> 
> diff --git a/tools/libxl/libxl_dom_save.c b/tools/libxl/libxl_dom_save.c
> index 72b61c7..18946c5 100644
> --- a/tools/libxl/libxl_dom_save.c
> +++ b/tools/libxl/libxl_dom_save.c
> @@ -404,6 +404,7 @@ void libxl__domain_save(libxl__egc *egc, 
> libxl__domain_save_state *dss)
>      dss->sws.ao  = dss->ao;
>      dss->sws.dss = dss;
>      dss->sws.fd  = dss->fd;
> +    dss->sws.back_channel = false;
>      dss->sws.completion_callback = stream_done;
>  
>      libxl__stream_write_start(egc, &dss->sws);
> diff --git a/tools/libxl/libxl_internal.h b/tools/libxl/libxl_internal.h
> index 3d3e8e8..e02e554 100644
> --- a/tools/libxl/libxl_internal.h
> +++ b/tools/libxl/libxl_internal.h
> @@ -3044,6 +3044,7 @@ struct libxl__stream_write_state {
>      libxl__ao *ao;
>      libxl__domain_save_state *dss;
>      int fd;
> +    bool back_channel;
>      void (*completion_callback)(libxl__egc *egc,
>                                  libxl__stream_write_state *sws,
>                                  int rc);
> diff --git a/tools/libxl/libxl_stream_write.c 
> b/tools/libxl/libxl_stream_write.c
> index f6ea55d..5379126 100644
> --- a/tools/libxl/libxl_stream_write.c
> +++ b/tools/libxl/libxl_stream_write.c
> @@ -49,6 +49,13 @@
>   *  - if (hvm)
>   *      - Emulator context record
>   *  - Checkpoint end record
> + *
> + * For back channel stream:
> + * - libxl__stream_write_start()
> + *    - Set up the stream to running state
> + *
> + * - Add a new API to write the record. When the record is written
> + *   out, call stream->checkpoint_callback() to return.

What does this mean? Which new API?

>   */
>  
>  /* Success/error/cleanup handling. */
> @@ -225,6 +232,15 @@ void libxl__stream_write_start(libxl__egc *egc,
>  
>      stream->running = true;
>  
> +    dc->ao        = ao;
> +    dc->readfd    = -1;
> +    dc->copywhat  = "save v2 stream";
> +    dc->writefd   = stream->fd;
> +    dc->maxsz     = -1;
> +
> +    if (stream->back_channel)
> +        return;
> +

There seems to be very subtle change of behaviour.

Before this patch, the dc->* are not set until the emulator check is
done. With this path, it is possible in the normal case some of the
fields are initialised in the error path.

I think this is OK given the callbacks in the upper layer and in
the writer don't rely on those fields to clean up. Just one thing to
note.

That said, I suggest you move all initialisation of dc->* in one place.

>      if (dss->type == LIBXL_DOMAIN_TYPE_HVM) {
>          stream->device_model_version =
>              libxl__device_model_version_running(gc, dss->domid);
> @@ -249,12 +265,7 @@ void libxl__stream_write_start(libxl__egc *egc,
>          stream->emu_sub_hdr.index = 0;
>      }
>  
> -    dc->ao        = ao;
> -    dc->readfd    = -1;
>      dc->writewhat = "stream header";
> -    dc->copywhat  = "save v2 stream";
> -    dc->writefd   = stream->fd;
> -    dc->maxsz     = -1;
>      dc->callback  = stream_header_done;
>  
>      rc = libxl__datacopier_start(dc);
> @@ -279,6 +290,7 @@ void libxl__stream_write_start_checkpoint(libxl__egc *egc,
>  {
>      assert(stream->running);
>      assert(!stream->in_checkpoint);
> +    assert(!stream->back_channel);
>      stream->in_checkpoint = true;
>  
>      write_emulator_xenstore_record(egc, stream);
> @@ -590,7 +602,9 @@ static void stream_done(libxl__egc *egc,
>          libxl__carefd_close(stream->emu_carefd);
>      free(stream->emu_body);
>  
> -    check_all_finished(egc, stream, rc);
> +    if (!stream->back_channel)
> +        /* back channel stream doesn't have save helper */
> +        check_all_finished(egc, stream, rc);

Though it doesn't have helper, do you not need to check if the back
channel stream itself is OK? The comment itself doesn't seem to justify
this change.

Wei.

>  }
>  
>  static void checkpoint_done(libxl__egc *egc,
> -- 
> 2.5.0
> 
> 
> 
> 
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