Re: [Xen-devel] [PATCH v5 2/5] libxl: add query function for backend support by device model

[Wei Liu <wei.liu2@xxxxxxxxxx>]

On Wed, Mar 30, 2016 at 02:05:55PM +0200, Juergen Gross wrote:
> Add a function to query whether the device model is supporting a
> specific backend type. The device model is writing the supported
> backend types to Xenstore on startup. The new query function checks
> for the appropriate entry to be present.
> 
> As not all versions of qemu are capable to indicate support of
> specific backends the query function is to be called with an indicator
> whether the default return value should be "supported" (in case qemu
> doesn't know set any support information) or "not supported".
> 
> Signed-off-by: Juergen Gross <jgross@xxxxxxxx>

The code itself looks straightforward enough.

But note that this is a new protocol that needs to be supported
essentially forever. I've CC QEMU maintainers for their input.

This also means even if we get this in for 4.7 we can't essentially
benefit from it until this protocol is implemented in upstream QEMU. At
this point I don't think I would take this particular patch for 4.7.

(I haven't checked if the rest depends on this one though)

Wei.

> ---
>  tools/libxl/libxl_dm.c       | 19 +++++++++++++++++++
>  tools/libxl/libxl_internal.h |  5 +++++
>  2 files changed, 24 insertions(+)
> 
> diff --git a/tools/libxl/libxl_dm.c b/tools/libxl/libxl_dm.c
> index 0d88c37..7d9abbe 100644
> --- a/tools/libxl/libxl_dm.c
> +++ b/tools/libxl/libxl_dm.c
> @@ -1920,6 +1920,25 @@ out:
>          device_model_spawn_outcome(egc, dmss, rc);
>  }
>  
> +bool libxl__query_qemu_backend(libxl__gc *gc, uint32_t domid,
> +                               uint32_t backend_id, const char *type, bool 
> def)
> +{
> +    char *path;
> +    char **dir;
> +    unsigned int n;
> +
> +    path = GCSPRINTF("%s/device-model/%u/backends",
> +                     libxl__xs_get_dompath(gc, backend_id), domid);
> +    dir = libxl__xs_directory(gc, XBT_NULL, path, &n);
> +    if (!dir)
> +        return def;
> +
> +    path = GCSPRINTF("%s/device-model/%u/backends/%s",
> +                     libxl__xs_get_dompath(gc, backend_id), domid, type);
> +    dir = libxl__xs_directory(gc, XBT_NULL, path, &n);
> +
> +    return !!dir;
> +}
>  
>  static void device_model_confirm(libxl__egc *egc, libxl__spawn_state *spawn,
>                                   const char *xsdata)
> diff --git a/tools/libxl/libxl_internal.h b/tools/libxl/libxl_internal.h
> index fc7bdab..c06ffc0 100644
> --- a/tools/libxl/libxl_internal.h
> +++ b/tools/libxl/libxl_internal.h
> @@ -1617,6 +1617,11 @@ _hidden const char 
> *libxl__domain_device_model(libxl__gc *gc,
>                                          const libxl_domain_build_info *info);
>  _hidden int libxl__need_xenpv_qemu(libxl__gc *gc,
>                                     libxl_domain_config *d_config);
> +_hidden bool libxl__query_qemu_backend(libxl__gc *gc,
> +                                       uint32_t domid,
> +                                       uint32_t backend_id,
> +                                       const char *type,
> +                                       bool def);
>  
>  /*
>   * This function will fix reserved device memory conflict
> -- 
> 2.6.2
> 

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