On Mon, 16 Dec 2019 12:07:42 +0100
Toke Høiland-Jørgensen <toke@xxxxxxxxxx> wrote:
> When attaching XDP programs, userspace can set flags to request the attach
> mode (generic/SKB mode, driver mode or hw offloaded mode). If no such flags
> are requested, the kernel will attempt to attach in driver mode, and then
> silently fall back to SKB mode if this fails.
>
> The silent fallback is a major source of user confusion, as users will try
> to load a program on a device without XDP support, and instead of an error
> they will get the silent fallback behaviour, not notice, and then wonder
> why performance is not what they were expecting.
>
> In an attempt to combat this, let's switch all the samples to default to
> explicitly requesting driver-mode attach. As part of this, ensure that all
> the userspace utilities have a switch to enable SKB mode. For those that
> have a switch to request driver mode, keep it but turn it into a no-op.
>
> Signed-off-by: Toke Høiland-Jørgensen <toke@xxxxxxxxxx>
> ---
I agree, that this is a good way forward.
What is the observed behavior / error-message after this change?
I wanted to test this myself, but compiling samples/bpf/ is breaking
(again) on my system...
> diff --git a/samples/bpf/xdp1_user.c b/samples/bpf/xdp1_user.c
> index 3e553eed95a7..38a8852cb57f 100644
> --- a/samples/bpf/xdp1_user.c
> +++ b/samples/bpf/xdp1_user.c
> @@ -98,7 +98,7 @@ int main(int argc, char **argv)
> xdp_flags |= XDP_FLAGS_SKB_MODE;
> break;
> case 'N':
> - xdp_flags |= XDP_FLAGS_DRV_MODE;
> + /* default, set below */
> break;
> case 'F':
> xdp_flags &= ~XDP_FLAGS_UPDATE_IF_NOEXIST;
> @@ -109,6 +109,9 @@ int main(int argc, char **argv)
> }
> }
>
> + if (!(xdp_flags & XDP_FLAGS_SKB_MODE))
> + xdp_flags |= XDP_FLAGS_DRV_MODE;
> +
> if (optind == argc) {
> usage(basename(argv[0]));
> return 1;
--
Best regards,
Jesper Dangaard Brouer
MSc.CS, Principal Kernel Engineer at Red Hat
LinkedIn: http://www.linkedin.com/in/brouer
