Re: Bad XDP performance with mlx5

[Tom Barbette <barbette@xxxxxx>]

CCing mlx5 maintainers and commiters of bce2b2b. TLDK: there is a huge 
CPU increase on CX5 when introducing a XDP program. See 
https://www.youtube.com/watch?v=o5hlJZbN4Tk&feature=youtu.be around 
0:40. We're talking something like 15% while it's near 0 for other 
drivers. The machine is a recent Skylake. For us it makes XDP unusable. 
Is that a known problem?

I wonder if it doesn't simply come from mlx5/en_main.c:

rq->buff.map_dir = rq->xdp_prog ? DMA_BIDIRECTIONAL : DMA_FROM_DEVICE;

Which would be inline from my observation that memory access seems 
heavier. I guess this is for the XDP_TX case.

If this is indeed the problem. Any chance we can:
a) detect automatically that a program will not return XDP_TX (I'm not 
quite sure about what the BPF limitations allow to guess in advance) or
b) add a flag to such as XDP_FLAGS_NO_TX to avoid such hit in 
performance when not needed?

Thanks,

Tom