Re: [RFC] avoid indirect calls for DMA direct mappings v2

From: Jesper Dangaard Brouer
Date: Sat Dec 08 2018 - 11:07:01 EST

On Fri, 7 Dec 2018 11:07:05 -0800
Christoph Hellwig <hch@xxxxxx> wrote:

> Hi all,
> a while ago Jesper reported major performance regressions due to the
> spectre v2 mitigations in his XDP forwarding workloads. A large part
> of that is due to the DMA mapping API indirect calls.
> It turns out that the most common implementation of the DMA API is the
> direct mapping case, and now that we have merged almost all duplicate
> implementations of that into a single generic one is easily feasily to
> direct calls for this fast path.
> This series adds consolidate the DMA mapping code by merging the
> swiotlb case into the dma direct case, and then treats NULL dma_ops
> as an indicator that that we should directly call the direct mapping
> case. This recovers a large part of the retpoline induces XDP slowdown.
> This works is based on the dma-mapping tree, so you probably want to
> want this git tree for testing:
> git:// dma-direct-calls.2
> Gitweb:

You can add my:
Tested-by: Jesper Dangaard Brouer <brouer@xxxxxxxxxx>
Acked-by: Jesper Dangaard Brouer <brouer@xxxxxxxxxx>

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:

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).

Best regards,
Jesper Dangaard Brouer
MSc.CS, Principal Kernel Engineer at Red Hat