Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Content-Type: text/plain;
        charset=us-ascii
Mime-Version: 1.0 (Mac OS X Mail 12.1 \(3445.101.1\))
Subject: Re: [RFC] avoid indirect calls for DMA direct mappings
From:   Nadav Amit <nadav.amit@gmail.com>
In-Reply-To: <20181206184351.4d9ece54@redhat.com>
Date:   Thu, 6 Dec 2018 10:29:35 -0800
Cc:     Christoph Hellwig <hch@lst.de>,
        =?utf-8?Q?Toke_H=C3=B8iland-J=C3=B8rgensen?= <toke@toke.dk>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Ilias Apalodimas <ilias.apalodimas@linaro.org>,
        LKML <linux-kernel@vger.kernel.org>,
        iommu <iommu@lists.linux-foundation.org>,
        Robin Murphy <robin.murphy@arm.com>,
        Tariq Toukan <tariqt@mellanox.com>,
        Josh Poimboeuf <jpoimboe@redhat.com>
Content-Transfer-Encoding: quoted-printable
Message-Id: <A9F9D621-445E-4F3C-95FE-3963A3DAEF98@gmail.com>
References: <20181206153720.10702-1-hch@lst.de>
 <20181206184351.4d9ece54@redhat.com>
To:     Jesper Dangaard Brouer <brouer@redhat.com>
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

> On Dec 6, 2018, at 9:43 AM, Jesper Dangaard Brouer <brouer@redhat.com> =
wrote:
>=20
> On Thu,  6 Dec 2018 07:37:19 -0800
> Christoph Hellwig <hch@lst.de> wrote:
>=20
>> Hi all,
>>=20
>> 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.
>>=20
>> 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.
>>=20
>> 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.
>=20
> Full test report avail here:
> =
https://github.com/xdp-project/xdp-project/blob/master/areas/dma/dma01_tes=
t_hellwig_direct_dma.org
>=20
>=20
>> 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:
>>=20
>>=20
>>    git://git.infradead.org/users/hch/misc.git dma-direct-calls
>>=20
>> Gitweb:
>>=20
>>    =
http://git.infradead.org/users/hch/misc.git/shortlog/refs/heads/dma-direct=
-calls

Did you happen to see my RFC for "automatic" indirect call promotion?=20

	https://lkml.org/lkml/2018/10/18/175

I hope to get v1 based on Josh responses next week.