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SubjectRe: [RFC] avoid indirect calls for DMA direct mappings
On Thu,  6 Dec 2018 07:37:19 -0800
Christoph Hellwig <> 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 patch adds a check if we are using dma_direct_ops in each fast path
> DMA operation, and just uses a direct call instead. For the XDP workload
> this increases the number of packets per second from 7,438,283 to
> 9,610,088, so it provides a very significant speedup.

Full test report avail here:

> Note that the patch depends on a lot of work either queued up in the
> DMA mapping tree, or still out on the list from review, so to actually
> try the patch you probably want this git tree:
> git:// dma-direct-calls
> Gitweb:

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

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