On 8/10/20 12:28 PM, Harshitha Ramamurthy wrote:
> This patch adds a helper function called bpf_get_skb_hash to calculate
> the skb hash for a packet at the XDP layer. In the helper function,
Why? i.e., expected use case?
Pulling this from hardware when possible is better. e.g., Saeed's
hardware hints proposal includes it.
> a local skb is allocated and we populate the fields needed in the skb
> before calling skb_get_hash. To avoid memory allocations for each packet,
> we allocate an skb per CPU and use the same buffer for subsequent hash
> calculations on the same CPU.
>
> Signed-off-by: Harshitha Ramamurthy <harshitha.ramamurthy@xxxxxxxxx>
> ---
> include/uapi/linux/bpf.h | 8 ++++++
> net/core/filter.c | 50 ++++++++++++++++++++++++++++++++++
> tools/include/uapi/linux/bpf.h | 8 ++++++
> 3 files changed, 66 insertions(+)
>
> diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
> index b134e679e9db..25aa850c8a40 100644
> --- a/include/uapi/linux/bpf.h
> +++ b/include/uapi/linux/bpf.h
> @@ -3394,6 +3394,13 @@ union bpf_attr {
> * A non-negative value equal to or less than *size* on success,
> * or a negative error in case of failure.
> *
> + * u32 bpf_get_skb_hash(struct xdp_buff *xdp_md)
> + * Description
> + * Return the skb hash for the xdp context passed. This function
> + * allocates a temporary skb and populates the fields needed. It
> + * then calls skb_get_hash to calculate the skb hash for the packet.
> + * Return
> + * The 32-bit hash.
> */
> #define __BPF_FUNC_MAPPER(FN) \
> FN(unspec), \
> @@ -3538,6 +3545,7 @@ union bpf_attr {
> FN(skc_to_tcp_request_sock), \
> FN(skc_to_udp6_sock), \
> FN(get_task_stack), \
> + FN(get_skb_hash), \
> /* */
>
> /* integer value in 'imm' field of BPF_CALL instruction selects which helper
> diff --git a/net/core/filter.c b/net/core/filter.c
> index 7124f0fe6974..9f6ad7209b44 100644
> --- a/net/core/filter.c
> +++ b/net/core/filter.c
> @@ -3765,6 +3765,54 @@ static const struct bpf_func_proto bpf_xdp_redirect_map_proto = {
> .arg3_type = ARG_ANYTHING,
> };
>
> +static DEFINE_PER_CPU(struct sk_buff *, hash_skb);
> +
> +BPF_CALL_1(bpf_get_skb_hash, struct xdp_buff *, xdp)
> +{
> + void *data_end = xdp->data_end;
> + struct ethhdr *eth = xdp->data;
> + void *data = xdp->data;
> + unsigned long flags;
> + struct sk_buff *skb;
> + int nh_off, len;
> + u32 ret = 0;
> +
> + /* disable interrupts to get the correct skb pointer */
> + local_irq_save(flags);
> +
> + len = data_end - data;
> + skb = this_cpu_read(hash_skb);
> + if (!skb) {
> + skb = alloc_skb(len, GFP_ATOMIC);
> + if (!skb)
> + goto out;
> + this_cpu_write(hash_skb, skb);
> + }
> +
> + nh_off = sizeof(*eth);
vlans?
> + if (data + nh_off > data_end)
> + goto out;
> +
> + skb->data = data;
> + skb->head = data;
> + skb->network_header = nh_off;
> + skb->protocol = eth->h_proto;
> + skb->len = len;
> + skb->dev = xdp->rxq->dev;
> +
> + ret = skb_get_hash(skb);
static inline __u32 skb_get_hash(struct sk_buff *skb)
{
if (!skb->l4_hash && !skb->sw_hash)
__skb_get_hash(skb);
return skb->hash;
}
__skb_get_hash -> __skb_set_sw_hash -> __skb_set_hash which sets
sw_hash as a minimum, so it seems to me you will always be returning the
hash of the first packet since you do not clear relevant fields of the skb.
