[RFC] avoid indirect calls for DMA direct mappings v2

[Christoph Hellwig]

On Sat, Dec 08, 2018 at 05:06:48PM +0100, Jesper Dangaard Brouer wrote:
> You can add my:
>  Tested-by: Jesper Dangaard Brouer <brouer at redhat.com>
> or
>  Acked-by: Jesper Dangaard Brouer <brouer at redhat.com>
> 
> I'm very happy that you work on this.  And I've done micro-benchmark
> testing of the patchset (and branch dma-direct-calls), which I've made
> avail here:
>  https://github.com/xdp-project/xdp-project/blob/master/areas/dma/dma01_test_hellwig_direct_dma.org
> 
> My XDP performance is back, minus the BPF-indirect call, and
> net_rx_action napi->poll, and net_device->ndo_xdp_xmit calls.  I
> verified that manually disabling retpoline for these remaining netstack
> retpoline-calls restore the performance full (well minus 1.5 nanosec).

Thanks a lot for all the testing (and the initial report).  I'd love to
still gets this in for 4.21 if we can, but for that I'd need a lot
more folks to carefull review it, especially the swiotlb and ia64 bits.
So if we don't get that early next week I feel we might have to punt
it to the next merge window.