linux-kernel - Re: [RFC] avoid indirect calls for DMA direct mappings

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Date:   Thu, 6 Dec 2018 18:43:51 +0100
From:   Jesper Dangaard Brouer <brouer@...hat.com>
To:     Christoph Hellwig <hch@....de>
Cc:     iommu@...ts.linux-foundation.org,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Tariq Toukan <tariqt@...lanox.com>,
        Ilias Apalodimas <ilias.apalodimas@...aro.org>,
        Toke Høiland-Jørgensen 
        <toke@...e.dk>, Robin Murphy <robin.murphy@....com>,
        linux-kernel@...r.kernel.org, brouer@...hat.com
Subject: Re: [RFC] avoid indirect calls for DMA direct mappings

On Thu,  6 Dec 2018 07:37:19 -0800
Christoph Hellwig <hch@....de> 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:
 https://github.com/xdp-project/xdp-project/blob/master/areas/dma/dma01_test_hellwig_direct_dma.org


> 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://git.infradead.org/users/hch/misc.git dma-direct-calls
> 
> Gitweb:
> 
>     http://git.infradead.org/users/hch/misc.git/shortlog/refs/heads/dma-direct-calls
> 



-- 
Best regards,
  Jesper Dangaard Brouer
  MSc.CS, Principal Kernel Engineer at Red Hat
  LinkedIn: http://www.linkedin.com/in/brouer