From: Tom Murphy <tmurphy@arista.com>
To: Joerg Roedel <joro@8bytes.org>
Cc: iommu@lists.linux-foundation.org, Tom Murphy <murphyt7@tcd.ie>,
Will Deacon <will.deacon@arm.com>,
Robin Murphy <robin.murphy@arm.com>,
Marek Szyprowski <m.szyprowski@samsung.com>,
Kukjin Kim <kgene@kernel.org>,
Krzysztof Kozlowski <krzk@kernel.org>,
David Woodhouse <dwmw2@infradead.org>,
Andy Gross <andy.gross@linaro.org>,
David Brown <david.brown@linaro.org>,
Matthias Brugger <matthias.bgg@gmail.com>,
Rob Clark <robdclark@gmail.com>, Heiko Stuebner <heiko@sntech.de>,
Gerald Schaefer <gerald.schaefer@de.ibm.com>,
Thierry Reding <thierry.reding@gmail.com>,
Jonathan Hunter <jonathanh@nvidia.com>,
linux-kernel@vger.kernel.org,
linux-arm-kernel@lists.infradead.org,
linux-samsung-soc@vger.kernel.org, linux-arm-msm@vger.kernel.org,
linux-mediatek@lists.infradead.org,
linux-rockchip@lists.infradead.org, linux-s390@vger.kernel.org,
linux-tegra@vger.kernel.org
Subject: Re: [PATCH v3 0/4] iommu/amd: Convert the AMD iommu driver to the dma-iommu api
Date: Mon, 3 Jun 2019 12:27:37 +0100 [thread overview]
Message-ID: <CAPL0++72dekt=re1=sTWpCJtMX=mUOc3Jcq=9d1sr1QO25_zFA@mail.gmail.com> (raw)
In-Reply-To: <20190603105158.GL12745@8bytes.org>
On Mon, Jun 3, 2019 at 11:52 AM Joerg Roedel <joro@8bytes.org> wrote:
>
> Hi Tom,
>
> On Mon, May 06, 2019 at 07:52:02PM +0100, Tom Murphy wrote:
> > Convert the AMD iommu driver to the dma-iommu api. Remove the iova
> > handling and reserve region code from the AMD iommu driver.
>
> Thank you for your work on this! I appreciate that much, but I am not
> sure we are ready to make that move for the AMD and Intel IOMMU drivers
> yet.
>
> My main concern right now is that these changes will add a per-page
> table lock into the fast-path for dma-mapping operations. There has been
> much work in the past to remove all locking from these code-paths and
> make it scalable on x86.
Where is the locking introduced? intel doesn't use a lock in it's
iommu_map function:
https://github.com/torvalds/linux/blob/f2c7c76c5d0a443053e94adb9f0918fa2fb85c3a/drivers/iommu/intel-iommu.c#L5302
because it cleverly uses cmpxchg64 to avoid using locks:
https://github.com/torvalds/linux/blob/f2c7c76c5d0a443053e94adb9f0918fa2fb85c3a/drivers/iommu/intel-iommu.c#L900
And the locking in AMD's iommu_map function can be removed (and i have
removed it in my patch set) because it does that same thing as intel:
https://github.com/torvalds/linux/blob/f2c7c76c5d0a443053e94adb9f0918fa2fb85c3a/drivers/iommu/amd_iommu.c#L1486
Is there something I'm missing?
>
> The dma-ops implementations in the x86 IOMMU drivers have the benefit
> that they can call their page-table manipulation functions directly and
> without locks, because they can make the necessary assumptions. The
> IOMMU-API mapping/unmapping path can't make these assumptions because it
> is also used for non-DMA-API use-cases.
>
> So before we can move the AMD and Intel drivers to the generic DMA-API
> implementation we need to solve this problem to not introduce new
> scalability regressions.
>
> Regards,
>
> Joerg
>
prev parent reply other threads:[~2019-06-03 11:27 UTC|newest]
Thread overview: 19+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-05-06 18:52 [PATCH v3 0/4] iommu/amd: Convert the AMD iommu driver to the dma-iommu api Tom Murphy via iommu
2019-05-06 18:52 ` Tom Murphy
2019-05-06 18:52 ` [PATCH v3 2/4] iommu/dma-iommu: Handle deferred devices Tom Murphy
2019-05-06 18:52 ` Tom Murphy
2019-05-07 6:40 ` Christoph Hellwig
2019-05-07 6:40 ` Christoph Hellwig
2019-05-15 12:46 ` Tom Murphy
[not found] ` <20190506185207.31069-1-tmurphy-nzgTgzXrdUbQT0dZR+AlfA@public.gmane.org>
2019-05-06 18:52 ` [PATCH v3 1/4] iommu: Add gfp parameter to iommu_ops::map Tom Murphy via iommu
2019-05-06 18:52 ` Tom Murphy
2019-06-04 18:11 ` Robin Murphy
2019-06-04 20:55 ` Tom Murphy
2019-06-04 21:24 ` Rob Clark
2019-06-05 5:50 ` Christoph Hellwig
2019-05-06 18:52 ` [PATCH v3 3/4] iommu/dma-iommu: Use the dev->coherent_dma_mask Tom Murphy via iommu
2019-05-06 18:52 ` Tom Murphy
2019-05-06 18:52 ` [PATCH v3 4/4] iommu/amd: Convert AMD iommu driver to the dma-iommu api Tom Murphy
2019-05-06 18:52 ` Tom Murphy
2019-06-03 10:51 ` [PATCH v3 0/4] iommu/amd: Convert the " Joerg Roedel
2019-06-03 11:27 ` Tom Murphy [this message]
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