Re: [Xen-devel] [PATCH 1/9] libxl: disks: Make LIBXL_DISK_BACKEND_UNKNOWN work

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Thu, 2011-06-02 at 18:55 +0100, Ian Jackson wrote:
> libxl callers should not be required to specify the backend if they
> don't want to.  So LIBXL_DISK_BACKEND_UNKNOWN should instruct libxl to
> choose the backend for itself.
> 
> For now, we do this by simply treating it the same as BACKEND_TAP,
> which itself falls back to other methods.
> 
> More thorough fixes for the disk handling will follow.
> 
> Signed-off-by: Ian Jackson <ian.jackson@xxxxxxxxxxxxx>
> ---
>  tools/libxl/libxl.c |   17 ++++++++++++++++-
>  1 files changed, 16 insertions(+), 1 deletions(-)
> 
> diff --git a/tools/libxl/libxl.c b/tools/libxl/libxl.c
> index c21cfe7..58df99c 100644
> --- a/tools/libxl/libxl.c
> +++ b/tools/libxl/libxl.c
> @@ -991,6 +991,21 @@ int libxl_device_disk_add(libxl_ctx *ctx, uint32_t 
> domid, libxl_device_disk *dis
>      device.domid = domid;
>      device.kind = DEVICE_VBD;
>  
> +
> +    /*
> +     * Fixing the incoming backend type to try to decide on which
> +     * backend to use.  Unfortunately at the moment this code is
> +     * utterly broken, but it more or less works.
> +     */
> +
> +    /*
> +     * Backend type UNKNOWN should mean "caller does not want to specify",
> +     * not "break pointlessely".  (Callers should not be required to
                              ^ no e here.

> +     * specify the backend if they don't want to.)
> +     */
> +    if (disk->backend == LIBXL_DISK_BACKEND_UNKNOWN)
> +        disk->backend = LIBXL_DISK_BACKEND_TAP;
> +
>      /* If blktap is not available then fallback to qdisk */
>      if (disk->backend == LIBXL_DISK_BACKEND_TAP && 
> !libxl__blktap_enabled(&gc))
>          disk->backend = LIBXL_DISK_BACKEND_QDISK;
> @@ -1138,6 +1153,7 @@ char * libxl_device_disk_local_attach(libxl_ctx *ctx, 
> libxl_device_disk *disk)
>              dev = disk->pdev_path;
>              break;
>          case LIBXL_DISK_BACKEND_TAP:
> +        case LIBXL_DISK_BACKEND_UNKNOWN:

I think this should be treated the same as you did above, by adding an
explicit fallback before the switch statement, for clarity.

Ian.

>              if (disk->format == LIBXL_DISK_FORMAT_VHD ||
>                  disk->format == LIBXL_DISK_FORMAT_RAW)
>              {
> @@ -1175,7 +1191,6 @@ char * libxl_device_disk_local_attach(libxl_ctx *ctx, 
> libxl_device_disk *disk)
>                  disk->pdev_path);
>              dev = disk->pdev_path;
>              break;
> -        case LIBXL_DISK_BACKEND_UNKNOWN:
>          default:
>              LIBXL__LOG(ctx, LIBXL__LOG_ERROR, "unrecognized disk backend "
>                  "type: %d", disk->backend);



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