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

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Date:   Tue, 27 Mar 2018 17:49:50 -0700
From:   William Tu <u9012063@...il.com>
To:     Tushar Dave <tushar.n.dave@...cle.com>
Cc:     Alexander Duyck <alexander.duyck@...il.com>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        Björn Töpel <bjorn.topel@...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>,
        jeffrey.b.shaw@...el.com, ferruh.yigit@...el.com,
        qi.z.zhang@...el.com
Subject: Re: [RFC PATCH 00/24] Introducing AF_XDP support

> Indeed. Intel iommu has least effect on RX because of premap/recycle.
> But TX dma map and unmap is really expensive!
>
>>
>> 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.
>
>
> Yeah thats what I would say, If you really want to use intel iommu and
> don't want to hit by performance , use 'iommu=pt'.
>
> Good to have confirmation from you Alex. Thanks.
>

Thanks for the suggestion! Update my performance number:

without iommu=pt (posted before)
Benchmark   XDP_SKB
rxdrop      2.3 Mpps
txpush     1.05 Mpps
l2fwd        0.90 Mpps

with iommu=pt (new)
Benchmark   XDP_SKB
rxdrop      2.24 Mpps
txpush     1.54 Mpps
l2fwd        1.23 Mpps

TX indeed shows better rate, while RX remains.
William