netdev - Re: [RFC PATCH 00/24] Introducing AF

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Date:   Tue, 27 Mar 2018 08:30:57 +0200
From:   Björn Töpel <bjorn.topel@...il.com>
To:     Alexander Duyck <alexander.duyck@...il.com>
Cc:     Tushar Dave <tushar.n.dave@...cle.com>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        William Tu <u9012063@...il.com>,
        "Karlsson, Magnus" <magnus.karlsson@...el.com>,
        Alexander Duyck <alexander.h.duyck@...el.com>,
        John Fastabend <john.fastabend@...il.com>,
        Alexei Starovoitov <ast@...com>,
        Willem de Bruijn <willemdebruijn.kernel@...il.com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>,
        Björn Töpel <bjorn.topel@...el.com>,
        michael.lundkvist@...csson.com,
        "Brandeburg, Jesse" <jesse.brandeburg@...el.com>,
        Anjali Singhai Jain <anjali.singhai@...el.com>,
        "Shaw, Jeffrey B" <jeffrey.b.shaw@...el.com>,
        "Yigit, Ferruh" <ferruh.yigit@...el.com>,
        "Zhang, Qi Z" <qi.z.zhang@...el.com>
Subject: Re: [RFC PATCH 00/24] Introducing AF_XDP support

2018-03-27 1:03 GMT+02:00 Alexander Duyck <alexander.duyck@...il.com>:
> On Mon, Mar 26, 2018 at 3:54 PM, Tushar Dave <tushar.n.dave@...cle.com> wrote:
[...]
>>
>> Whats the implication here. Should IOMMU be disabled?
>> I'm asking because I do see a huge difference while running pktgen test for
>> my performance benchmarks, with and without intel_iommu.
>>
>>
>> -Tushar
>
> For the Intel parts the IOMMU can be expensive primarily for Tx, since
> it should have minimal impact if the Rx pages are pinned/recycled. I
> am assuming the same is true here for AF_XDP, Bjorn can correct me if
> I am wrong.
>

For the non-zc case the DMA mapping is done in the Tx fast path, so
there, as Alex says, you'll definitely see a performance penalty. For
Rx the page-recycle mechanism (Intel drivers) usally avoids doing any
DMA mappings in the fast-path.

As for AF_XDP zerocopy mode, we do the DMA mapping up front (avoiding
the single-use mappings), to avoid that performance hit. Keep in mind,
though, that the IOTLB is still in play, and usually performs worse
under pressure, than the non-IOMMU case.

> Basically the IOMMU can make creating/destroying a DMA mapping really
> expensive. The easiest way to work around it in the case of the Intel
> IOMMU is to boot with "iommu=pt" which will create an identity mapping
> for the host. The downside is though that you then have the entire
> system accessible to the device unless a new mapping is created for it
> by assigning it to a new IOMMU domain.
>



> Thanks.
>
> - Alex