On Tue, 5 Jan 2021 18:43:08 +0100 Marek Behún <ka...@kernel.org> wrote:
> On Tue, 5 Jan 2021 18:24:37 +0100 > Sven Auhagen <sven.auha...@voleatech.de> wrote: > > > On Tue, Jan 05, 2021 at 06:19:21PM +0100, Marek Behún wrote: > > > Currently mvpp2_xdp_setup won't allow attaching XDP program if > > > mtu > ETH_DATA_LEN (1500). > > > > > > The mvpp2_change_mtu on the other hand checks whether > > > MVPP2_RX_PKT_SIZE(mtu) > MVPP2_BM_LONG_PKT_SIZE. > > > > > > These two checks are semantically different. > > > > > > Moreover this limit can be increased to MVPP2_MAX_RX_BUF_SIZE, since in > > > mvpp2_rx we have > > > xdp.data = data + MVPP2_MH_SIZE + MVPP2_SKB_HEADROOM; > > > xdp.frame_sz = PAGE_SIZE; > > > > > > Change the checks to check whether > > > mtu > MVPP2_MAX_RX_BUF_SIZE > > > > Hello Marek, > > > > in general, XDP is based on the model, that packets are not bigger > > than 1500. This is WRONG. The XDP design/model (with PAGE_SIZE 4096) allows MTU to be 3506 bytes. This comes from: * 4096 = frame_sz = PAGE_SIZE * -256 = reserved XDP_PACKET_HEADROOM * -320 = reserved tailroom with sizeof(skb_shared_info) * - 14 = Ethernet header size as MTU value is L3 4096-256-320-14 = 3506 bytes Depending on driver memory layout choices this can (of-cause) be lower. > > I am not sure if that has changed, I don't believe Jumbo Frames are > > upstreamed yet. This is unrelated to this patch, but Lorenzo and Eelco is assigned to work on this. > > You are correct that the MVPP2 driver can handle bigger packets > > without a problem but if you do XDP redirect that won't work with > > other drivers and your packets will disappear. > This statement is too harsh. The XDP layer will not do (IP-level) fragmentation for you. Thus, if you redirect/transmit frames out another interface with lower MTU than the frame packet size then the packet will of-cause be dropped (the drop counter is unfortunately not well defined). This is pretty standard behavior. This is why I'm proposing the BPF-helper bpf_check_mtu(). To allow the BPF-prog to check the MTU before doing the redirect. > At least 1508 is required when I want to use XDP with a Marvell DSA > switch: the DSA header is 4 or 8 bytes long there. > > The DSA driver increases MTU on CPU switch interface by this length > (on my switches to 1504). > > So without this I cannot use XDP with mvpp2 with a Marvell switch with > default settings, which I think is not OK. > > Since with the mvneta driver it works (mvneta checks for > MVNETA_MAX_RX_BUF_SIZE rather than ETH_DATA_LEN), I think it should also work > with mvpp2. I think you patch makes perfect sense. -- Best regards, Jesper Dangaard Brouer MSc.CS, Principal Kernel Engineer at Red Hat LinkedIn: http://www.linkedin.com/in/brouer
