Re: [PATCH] libxl: force netback to wait for hotplug execution before connecting

[Julien Grall <julien@xxxxxxx>]

Hi,

On 24/01/2022 16:02, Roger Pau Monne wrote:
> By writing an empty "hotplug-status" xenstore node in the backend path
> libxl can force Linux netback to wait for hotplug script execution
> before proceeding to the 'connected' state.
I was actually chasing the same issue today :).

> This is required so that netback doesn't skip state 2 (InitWait) and
Technically netback never skip state 2 (otherwise it would always be 
reproducible). Instead, libxl may not be able to observe state 2 because 
receive a watch is asynchronous and doesn't contain the value of the 
node. So the backend may have moved to Connected before the state is read.
> thus blocks libxl waiting for such state in order to launch the
> hotplug script (see libxl__wait_device_connection).
>
> Reported-by: James Dingwall <james-xen@xxxxxxxxxxxxxx>
> Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
> Tested-by: James Dingwall <james-xen@xxxxxxxxxxxxxx>
I could easily reproduced by adding a sleep(1) before reading the key 
and use 'xl network-attach ...'.
Tested-by: Julien Grall <jgrall@xxxxxxxxxx>

> ---
> Cc: Wei Liu <wei.liu@xxxxxxxxxx>
> Cc: Paul Durrant <paul@xxxxxxx>
> ---
>   tools/libs/light/libxl_nic.c | 7 +++++++
>   1 file changed, 7 insertions(+)
>
> diff --git a/tools/libs/light/libxl_nic.c b/tools/libs/light/libxl_nic.c
> index 0b45469dca..0b9e70c9d1 100644
> --- a/tools/libs/light/libxl_nic.c
> +++ b/tools/libs/light/libxl_nic.c
> @@ -248,6 +248,13 @@ static int libxl__set_xenstore_nic(libxl__gc *gc, uint32_t 
> domid,
>       flexarray_append(ro_front, "mtu");
>       flexarray_append(ro_front, GCSPRINTF("%u", nic->mtu));
>   
> +    /*
> +     * Force backend to wait for hotplug script execution before switching to
> +     * connected state.
> +     */
> +    flexarray_append(back, "hotplug-status");
> +    flexarray_append(back, "");
> +
>       return 0;
>   }
>   
Cheers,

--
Julien Grall