Re: [PATCH] net: improve error message for missing netdev backend

[Christian Schoenebeck <qemu_oss@crudebyte.com>]

On Montag, 3. Oktober 2022 12:06:12 CEST Daniel P. Berrangé wrote:
> The current message when using '-net user...' with SLIRP disabled at
> compile time is:
> 
>   qemu-system-x86_64: -net user: Parameter 'type' expects a net backend type
> (maybe it is not compiled into this binary)

Is this intended as alternative to Marc-André's previous patch? If yes, then 
same applies here: what about people not passing any networking arg to QEMU? 
They would not get any error message at all, right?

https://lore.kernel.org/all/2973900.g0HVWOepMQ@silver/

> An observation is that we're using the 'netdev->type' field here which
> is an enum value, produced after QAPI has converted from its string
> form.
> 
> IOW, at this point in the code, we know that the user's specified
> type name was a valid network backend. The only possible scenario that
> can make the backend init function be NULL, is if support for that
> backend was disabled at build time. Given this, we don't need to caveat
> our error message with a 'maybe' hint, we can be totally explicit.
> 
> The use of QERR_INVALID_PARAMETER_VALUE doesn't really lend itself to
> user friendly error message text. Since this is not used to set a
> specific QAPI error class, we can simply stop using this pre-formatted
> error text and provide something better.
> 
> Thus the new message is:
> 
>   qemu-system-x86_64: -net user: network backend 'user' is not compiled into
> this binary

And why not naming the child, i.e. that QEMU was built without slirp?

> The case of passing 'hubport' for -net is also given a message reminding
> people they should have used -netdev/-nic instead, as this backend type
> is only valid for the modern syntax.
> 
> Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
> ---
> 
> NB, this does not make any difference to people who were relying on the
> QEMU built-in default hub that was created if you don't list any -net /
> -netdev / -nic argument, only those using explicit args.
> 
>  net/net.c | 18 +++++++++++-------
>  1 file changed, 11 insertions(+), 7 deletions(-)
> 
> diff --git a/net/net.c b/net/net.c
> index 2db160e063..8ddafacf13 100644
> --- a/net/net.c
> +++ b/net/net.c
> @@ -1036,19 +1036,23 @@ static int net_client_init1(const Netdev *netdev,
> bool is_netdev, Error **errp) if (is_netdev) {
>          if (netdev->type == NET_CLIENT_DRIVER_NIC ||
>              !net_client_init_fun[netdev->type]) {
> -            error_setg(errp, QERR_INVALID_PARAMETER_VALUE, "type",
> -                       "a netdev backend type");
> +            error_setg(errp, "network backend '%s' is not compiled into
> this binary", +                       NetClientDriver_str(netdev->type));
>              return -1;
>          }
>      } else {
>          if (netdev->type == NET_CLIENT_DRIVER_NONE) {
>              return 0; /* nothing to do */
>          }
> -        if (netdev->type == NET_CLIENT_DRIVER_HUBPORT ||
> -            !net_client_init_fun[netdev->type]) {
> -            error_setg(errp, QERR_INVALID_PARAMETER_VALUE, "type",
> -                       "a net backend type (maybe it is not compiled "
> -                       "into this binary)");
> +        if (netdev->type == NET_CLIENT_DRIVER_HUBPORT) {
> +            error_setg(errp, "network backend '%s' is only supported with
> -netdev/-nic", +                       NetClientDriver_str(netdev->type));
> +            return -1;
> +        }
> +
> +        if (!net_client_init_fun[netdev->type]) {
> +            error_setg(errp, "network backend '%s' is not compiled into
> this binary", +                       NetClientDriver_str(netdev->type));
>              return -1;
>          }