Re: [RFC bpf-next 0/4] Add XDP rx hw hints support performing XDP_REDIRECT

[Lorenzo Bianconi <lorenzo.bianconi@redhat.com>]

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> Lorenzo Bianconi <lorenzo.bianconi@redhat.com> writes:
> 
> >> 
> >> 
> >> On 21/09/2024 22.17, Alexander Lobakin wrote:
> >> > From: Lorenzo Bianconi <lorenzo@kernel.org>
> >> > 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 = &reg->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
> 

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