> Lorenzo Bianconi <lorenzo.bianconi@xxxxxxxxxx> writes: > > >> > >> > >> On 21/09/2024 22.17, Alexander Lobakin wrote: > >> > From: Lorenzo Bianconi <lorenzo@xxxxxxxxxx> > >> > Date: Sat, 21 Sep 2024 18:52:56 +0200 > >> > > >> > > This series introduces the xdp_rx_meta struct in the xdp_buff/xdp_frame > >> > > >> > &xdp_buff is on the stack. > >> > &xdp_frame consumes headroom. > >> > > >> > IOW they're size-sensitive and putting metadata directly there might > >> > play bad; if not now, then later. > >> > > >> > Our idea (me + Toke) was as follows: > >> > > >> > - new BPF kfunc to build generic meta. If called, the driver builds a > >> > generic meta with hash, csum etc., in the data_meta area. > >> > >> I do agree that it should be the XDP prog (via a new BPF kfunc) that > >> decide if xdp_frame should be updated to contain a generic meta struct. > >> *BUT* I think we should use the xdp_frame area, and not the > >> xdp->data_meta area. > > > > ack, I will add a new kfunc for it. > > > >> > >> A details is that I think this kfunc should write data directly into > >> xdp_frame area, even then we are only operating on the xdp_buff, as we > >> do have access to the area xdp_frame will be created in. > > > > this would avoid to copy it when we convert from xdp_buff to xdp_frame, nice :) > > > >> > >> > >> When using data_meta area, then netstack encap/decap needs to move the > >> data_meta area (extra cycles). The xdp_frame area (live in top) don't > >> have this issue. > >> > >> It is easier to allow xdp_frame area to survive longer together with the > >> SKB. Today we "release" this xdp_frame area to be used by SKB for extra > >> headroom (see xdp_scrub_frame). I can imagine that we can move SKB > >> fields to this area, and reduce the size of the SKB alloc. (This then > >> becomes the mini-SKB we discussed a couple of years ago). > >> > >> > >> > Yes, this also consumes headroom, but only when the corresponding func > >> > is called. Introducing new fields like you're doing will consume it > >> > unconditionally; > >> > >> We agree on the kfunc call marks area as consumed/in-use. We can extend > >> xdp_frame statically like Lorenzo does (with struct xdp_rx_meta), but > >> xdp_frame->flags can be used for marking this area as used or not. > > > > the only downside with respect to a TLV approach would be to consume all the > > xdp_rx_meta as soon as we set a single xdp rx hw hint in it, right? > > The upside is it is easier and it requires less instructions. > > FYI, we also had a discussion related to this at LPC on Friday, in this > session: https://lpc.events/event/18/contributions/1935/ Hi Toke, thx for the pointer > > The context here was that Arthur and Jakub want to also support extended > rich metadata all the way through the SKB path, and are looking at the > same area used for XDP metadata to store it. So there's a need to manage > both the kernel's own usage of that area, and userspace/BPF usage of it. it would be cool if we can collaborate on it. > > I'll try to summarise some of the points of that discussion (all > interpretations are my own, of course): > > - We want something that can be carried with a frame all the way from > the XDP layer, through all SKB layers and to userspace (to replace the > use of skb->mark for this purpose). > > - We want different applications running on the system (of which the > kernel itself if one, cf this discussion) to be able to share this > field, without having to have an out of band registry (like a Github > repository where applications can agree on which bits to use). Which > probably means that the kernel needs to be in the loop somehow to > explicitly allocate space in the metadata area and track offsets. > > - Having an explicit API to access this from userspace, without having > to go through BPF (i.e., a socket- or CMSG-based API) would be useful. > > > The TLV format was one of the suggestions in Arthur and Jakub's talk, > but AFAICT, there was not a lot of enthusiasm about this in the room > (myself included), because of the parsing overhead and complexity. I > believe the alternative that was seen as most favourable was a map > lookup-style API, where applications can request a metadata area of > arbitrary size and get an ID assigned that they can then use to set/get > values in the data path. > > So, sketching this out, this could be realised by something like: > > /* could be called from BPF, or through netlink or sysfs; may fail, if > * there is no more space > */ > int metadata_id = register_packet_metadata_field(sizeof(struct my_meta)); > > The ID is just an opaque identifier that can then be passed to > getter/setter functions (for both SKB and XDP), like: > > ret = bpf_set_packet_metadata_field(pkt, metadata_id, > &my_meta_value, sizeof(my_meta_value)) > > ret = bpf_get_packet_metadata_field(pkt, metadata_id, > &my_meta_value, sizeof(my_meta_value)) > > > On the kernel side, the implementation would track registered fields in > a global structure somewhere, say: > > struct pkt_metadata_entry { > int id; > u8 sz; > u8 offset; > u8 bit; > }; > > struct pkt_metadata_registry { /* allocated as a system-wide global */ > u8 num_entries; > u8 total_size; > struct pkt_metadata_entry entries[MAX_ENTRIES]; > }; > > struct xdp_rx_meta { /* at then end of xdp_frame */ > u8 sz; /* set to pkt_metadata_registry->total_size on alloc */ > u8 fields_set; /* bitmap of fields that have been set, see below */ > u8 data[]; > }; > > int register_packet_metadata_field(u8 size) { > struct pkt_metadata_registry *reg = get_global_registry(); > struct pkt_metadata_entry *entry; > > if (size + reg->total_size > MAX_METADATA_SIZE) > return -ENOSPC; > > entry = ®->entries[reg->num_entries++]; > entry->id = assign_id(); > entry->sz = size; > entry->offset = reg->total_size; > entry->bit = reg->num_entries - 1; > reg->total_size += size; > > return entry->id; > } > > int bpf_set_packet_metadata_field(struct xdp_frame *frm, int id, void > *value, size_t sz) > { > struct pkt_metadata_entry *entry = get_metadata_entry_by_id(id); > > if (!entry) > return -ENOENT; > > if (entry->sz != sz) > return -EINVAL; /* user error */ > > if (frm->rx_meta.sz < entry->offset + sz) > return -EFAULT; /* entry allocated after xdp_frame was initialised */ > > memcpy(&frm->rx_meta.data + entry->offset, value, sz); > frm->rx_meta.fields_set |= BIT(entry->bit); > > return 0; > } > > int bpf_get_packet_metadata_field(struct xdp_frame *frm, int id, void > *value, size_t sz) > { > struct pkt_metadata_entry *entry = get_metadata_entry_by_id(id); > > if (!entry) > return -ENOENT; > > if (entry->sz != sz) > return -EINVAL; > > if (frm->rx_meta.sz < entry->offset + sz) > return -EFAULT; /* entry allocated after xdp_frame was initialised */ > > if (!(frm->rx_meta.fields_set & BIT(entry->bit))) > return -ENOENT; > > memcpy(value, &frm->rx_meta.data + entry->offset, sz); > > return 0; > } > > I'm hinting at some complications here (with the EFAULT return) that > needs to be resolved: there is no guarantee that a given packet will be > in sync with the current status of the registered metadata, so we need > explicit checks for this. If metadata entries are de-registered again > this also means dealing with holes and/or reshuffling the metadata > layout to reuse the released space (incidentally, this is the one place > where a TLV format would have advantages). I like this approach but it seems to me more suitable for 'sw' metadata (this is main Arthur and Jakub use case iiuc) where the userspace would enable/disable these functionalities system-wide. Regarding device hw metadata (e.g. checksum offload) I can see some issues since on a system we can have multiple NICs with different capabilities. If we consider current codebase, stmmac driver supports only rx timestamp, while mlx5 supports all of them. In a theoretical system with these two NICs, since pkt_metadata_registry is global system-wide, we will end-up with quite a lot of holes for the stmmac, right? (I am not sure if this case is relevant or not). In other words, we will end-up with a fixed struct for device rx hw metadata (like xdp_rx_meta). So I am wondering if we really need all this complexity for xdp rx hw metadata? Maybe we can start with a simple approach for xdp rx hw metadata putting the struct in xdp_frame as suggested by Jesper and covering the most common use-cases. We can then integrate this approach with Arthur/Jakub's solution without introducing any backward compatibility issue since these field are not visible to userspace. Regards, Lorenzo > > The nice thing about an API like this, though, is that it's extensible, > and the kernel itself can be just another consumer of it for the > metadata fields Lorenzo is adding in this series. I.e., we could just > pre-define some IDs for metadata vlan, timestamp etc, and use the same > functions as above from within the kernel to set and get those values; > using the registry, there could even be an option to turn those off if > an application wants more space for its own usage. Or, alternatively, we > could keep the kernel-internal IDs hardcoded and always allocated, and > just use the getter/setter functions as the BPF API for accessing them. > > -Toke >
Attachment:
signature.asc
Description: PGP signature
