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Date: Tue, 8 Nov 2022 13:54:41 -0800
From: Stanislav Fomichev <sdf@...gle.com>
To: Martin KaFai Lau <martin.lau@...ux.dev>
Cc: ast@...nel.org, daniel@...earbox.net, andrii@...nel.org,
song@...nel.org, yhs@...com, john.fastabend@...il.com,
kpsingh@...nel.org, haoluo@...gle.com, jolsa@...nel.org,
David Ahern <dsahern@...il.com>,
Jakub Kicinski <kuba@...nel.org>,
Willem de Bruijn <willemb@...gle.com>,
Jesper Dangaard Brouer <brouer@...hat.com>,
Anatoly Burakov <anatoly.burakov@...el.com>,
Alexander Lobakin <alexandr.lobakin@...el.com>,
Magnus Karlsson <magnus.karlsson@...il.com>,
Maryam Tahhan <mtahhan@...hat.com>, xdp-hints@...-project.net,
netdev@...r.kernel.org, bpf@...r.kernel.org
Subject: Re: [RFC bpf-next v2 06/14] xdp: Carry over xdp metadata into skb context
On Mon, Nov 7, 2022 at 2:02 PM Martin KaFai Lau <martin.lau@...ux.dev> wrote:
>
> On 11/3/22 8:25 PM, Stanislav Fomichev wrote:
> > > diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
> > index 59c9fd55699d..dba857f212d7 100644
> > --- a/include/linux/skbuff.h
> > +++ b/include/linux/skbuff.h
> > @@ -4217,9 +4217,13 @@ static inline bool skb_metadata_differs(const struct sk_buff *skb_a,
> > true : __skb_metadata_differs(skb_a, skb_b, len_a);
> > }
> >
> > +void skb_metadata_import_from_xdp(struct sk_buff *skb, size_t len);
> > +
> > static inline void skb_metadata_set(struct sk_buff *skb, u8 meta_len)
> > {
> > skb_shinfo(skb)->meta_len = meta_len;
> > + if (meta_len)
> > + skb_metadata_import_from_xdp(skb, meta_len);
> > }
> >
> [ ... ]
>
> > +struct xdp_to_skb_metadata {
> > + u32 magic; /* xdp_metadata_magic */
> > + u64 rx_timestamp;
> > +} __randomize_layout;
> > +
> > +struct bpf_patch;
> > +
>
> [ ... ]
>
> > +void skb_metadata_import_from_xdp(struct sk_buff *skb, size_t len)
> > +{
> > + struct xdp_to_skb_metadata *meta = (void *)(skb_mac_header(skb) - len);
> > +
> > + /* Optional SKB info, currently missing:
> > + * - HW checksum info (skb->ip_summed)
> > + * - HW RX hash (skb_set_hash)
> > + * - RX ring dev queue index (skb_record_rx_queue)
> > + */
> > +
> > + if (len != sizeof(struct xdp_to_skb_metadata))
> > + return;
> > +
> > + if (meta->magic != xdp_metadata_magic)
> > + return;
> > +
> > + if (meta->rx_timestamp) {
> > + *skb_hwtstamps(skb) = (struct skb_shared_hwtstamps){
> > + .hwtstamp = ns_to_ktime(meta->rx_timestamp),
> > + };
> > + }
> > +}
>
> Considering the metadata will affect the gro, should the meta be cleared after
> importing to the skb?
Yeah, good suggestion, will clear it here.
> [ ... ]
>
> > +/* Since we're not actually doing a call but instead rewriting
> > + * in place, we can only afford to use R0-R5 scratch registers.
> > + *
> > + * We reserve R1 for bpf_xdp_metadata_export_to_skb and let individual
> > + * metadata kfuncs use only R0,R4-R5.
> > + *
> > + * The above also means we _cannot_ easily call any other helper/kfunc
> > + * because there is no place for us to preserve our R1 argument;
> > + * existing R6-R9 belong to the callee.
> > + */
> > +void xdp_metadata_export_to_skb(const struct bpf_prog *prog, struct bpf_patch *patch)
> > +{
> > + u32 func_id;
> > +
> > + /*
> > + * The code below generates the following:
> > + *
> > + * void bpf_xdp_metadata_export_to_skb(struct xdp_md *ctx)
> > + * {
> > + * struct xdp_to_skb_metadata *meta;
> > + * int ret;
> > + *
> > + * ret = bpf_xdp_adjust_meta(ctx, -sizeof(*meta));
> > + * if (!ret)
> > + * return;
> > + *
> > + * meta = ctx->data_meta;
> > + * meta->magic = xdp_metadata_magic;
> > + * meta->rx_timestamp = bpf_xdp_metadata_rx_timestamp(ctx);
> > + * }
> > + *
> > + */
> > +
> > + bpf_patch_append(patch,
> > + /* r2 = ((struct xdp_buff *)r1)->data_meta; */
> > + BPF_LDX_MEM(BPF_DW, BPF_REG_2, BPF_REG_1,
> > + offsetof(struct xdp_buff, data_meta)),
> > + /* r3 = ((struct xdp_buff *)r1)->data; */
> > + BPF_LDX_MEM(BPF_DW, BPF_REG_3, BPF_REG_1,
> > + offsetof(struct xdp_buff, data)),
> > + /* if (data_meta != data) return;
> > + *
> > + * data_meta > data: xdp_data_meta_unsupported()
> > + * data_meta < data: already used, no need to touch
> > + */
> > + BPF_JMP_REG(BPF_JNE, BPF_REG_2, BPF_REG_3, S16_MAX),
> > +
> > + /* r2 -= sizeof(struct xdp_to_skb_metadata); */
> > + BPF_ALU64_IMM(BPF_SUB, BPF_REG_2,
> > + sizeof(struct xdp_to_skb_metadata)),
> > + /* r3 = ((struct xdp_buff *)r1)->data_hard_start; */
> > + BPF_LDX_MEM(BPF_DW, BPF_REG_3, BPF_REG_1,
> > + offsetof(struct xdp_buff, data_hard_start)),
> > + /* r3 += sizeof(struct xdp_frame) */
> > + BPF_ALU64_IMM(BPF_ADD, BPF_REG_3,
> > + sizeof(struct xdp_frame)),
> > + /* if (data-sizeof(struct xdp_to_skb_metadata) < data_hard_start+sizeof(struct xdp_frame)) return; */
> > + BPF_JMP_REG(BPF_JLT, BPF_REG_2, BPF_REG_3, S16_MAX),
> > +
> > + /* ((struct xdp_buff *)r1)->data_meta = r2; */
> > + BPF_STX_MEM(BPF_DW, BPF_REG_1, BPF_REG_2,
> > + offsetof(struct xdp_buff, data_meta)),
> > +
> > + /* *((struct xdp_to_skb_metadata *)r2)->magic = xdp_metadata_magic; */
> > + BPF_ST_MEM(BPF_W, BPF_REG_2,
> > + offsetof(struct xdp_to_skb_metadata, magic),
> > + xdp_metadata_magic),
> > + );
> > +
> > + /* r0 = bpf_xdp_metadata_rx_timestamp(ctx); */
> > + func_id = xdp_metadata_kfunc_id(XDP_METADATA_KFUNC_RX_TIMESTAMP);
> > + prog->aux->xdp_kfunc_ndo->ndo_unroll_kfunc(prog, func_id, patch);
> > +
> > + bpf_patch_append(patch,
> > + /* r2 = ((struct xdp_buff *)r1)->data_meta; */
> > + BPF_LDX_MEM(BPF_DW, BPF_REG_2, BPF_REG_1,
> > + offsetof(struct xdp_buff, data_meta)),
> > + /* *((struct xdp_to_skb_metadata *)r2)->rx_timestamp = r0; */
> > + BPF_STX_MEM(BPF_DW, BPF_REG_2, BPF_REG_0,
> > + offsetof(struct xdp_to_skb_metadata, rx_timestamp)),
>
> Can the xdp prog still change the metadata through xdp->data_meta? tbh, I am not
> sure it is solid enough by asking the xdp prog not to use the same random number
> in its own metadata + not to change the metadata through xdp->data_meta after
> calling bpf_xdp_metadata_export_to_skb().
What do you think the usecase here might be? Or are you suggesting we
reject further access to data_meta after
bpf_xdp_metadata_export_to_skb somehow?
If we want to let the programs override some of this
bpf_xdp_metadata_export_to_skb() metadata, it feels like we can add
more kfuncs instead of exposing the layout?
bpf_xdp_metadata_export_to_skb(ctx);
bpf_xdp_metadata_export_skb_hash(ctx, 1234);
...
> Does xdp_to_skb_metadata have a use case for XDP_PASS (like patch 7) or the
> xdp_to_skb_metadata can be limited to XDP_REDIRECT only?
XDP_PASS cases where we convert xdp_buff into skb in the drivers right
now usually have C code to manually pull out the metadata (out of hw
desc) and put it into skb.
So, currently, if we're calling bpf_xdp_metadata_export_to_skb() for
XDP_PASS, we're doing a double amount of work:
skb_metadata_import_from_xdp first, then custom driver code second.
In theory, maybe we should completely skip drivers custom parsing when
there is a prog with BPF_F_XDP_HAS_METADATA?
Then both xdp->skb paths (XDP_PASS+XDP_REDIRECT) will be bpf-driven
and won't require any mental work (plus, the drivers won't have to
care either in the future).
WDYT?
> > + );
> > +
> > + bpf_patch_resolve_jmp(patch);
> > +}
> > +
> > static int __init xdp_metadata_init(void)
> > {
> > + xdp_metadata_magic = get_random_u32() | 1;
> > return register_btf_kfunc_id_set(BPF_PROG_TYPE_XDP, &xdp_metadata_kfunc_set);
> > }
> > late_initcall(xdp_metadata_init);
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