netdev - Re: [net-next, PATCH 1/2, v3] net: socionext: different approach on DMA

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Date:   Mon, 1 Oct 2018 12:56:58 +0300
From:   Ilias Apalodimas <ilias.apalodimas@...aro.org>
To:     Jesper Dangaard Brouer <brouer@...hat.com>
Cc:     netdev@...r.kernel.org, jaswinder.singh@...aro.org,
        ard.biesheuvel@...aro.org, masami.hiramatsu@...aro.org,
        arnd@...db.de, bjorn.topel@...el.com, magnus.karlsson@...el.com,
        daniel@...earbox.net, ast@...nel.org,
        jesus.sanchez-palencia@...el.com, vinicius.gomes@...el.com,
        makita.toshiaki@....ntt.co.jp, Tariq Toukan <tariqt@...lanox.com>,
        Tariq Toukan <ttoukan.linux@...il.com>
Subject: Re: [net-next, PATCH 1/2, v3] net: socionext: different approach on
 DMA

> > #2: You have allocations on the XDP fast-path.
> > 
> > The REAL secret behind the XDP performance is to avoid allocations on
> > the fast-path.  While I just told you to use the page-allocator and
> > order-0 pages, this will actually kill performance.  Thus, to make this
> > fast, you need a driver local recycle scheme that avoids going through
> > the page allocator, which makes XDP_DROP and XDP_TX extremely fast.
> > For the XDP_REDIRECT action (which you seems to be interested in, as
> > this is needed for AF_XDP), there is a xdp_return_frame() API that can
> > make this fast.
> I had an initial implementation that did exactly that (that's why you the
> dma_sync_single_for_cpu() -> dma_unmap_single_attrs() is there). In the case 
> of AF_XDP isn't that introducing a 'bottleneck' though? I mean you'll feed fresh
> buffers back to the hardware only when your packets have been processed from
> your userspace application
Just a clarification here. This is the case if ZC is implemented. In my case
the buffers will be 'ok' to be passed back to the hardware once the use
userspace payload has been copied by xdp_do_redirect()

/Ilias