linux-kernel - Re: [PATCH v3 1/2] dma-direct: provide the ability to reserve per-numa CMA

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Message-ID: <20200723115527.GA31598@lst.de>
Date:   Thu, 23 Jul 2020 13:55:27 +0200
From:   Christoph Hellwig <hch@....de>
To:     "Song Bao Hua (Barry Song)" <song.bao.hua@...ilicon.com>
Cc:     Christoph Hellwig <hch@....de>,
        "m.szyprowski@...sung.com" <m.szyprowski@...sung.com>,
        "robin.murphy@....com" <robin.murphy@....com>,
        "will@...nel.org" <will@...nel.org>,
        "ganapatrao.kulkarni@...ium.com" <ganapatrao.kulkarni@...ium.com>,
        "catalin.marinas@....com" <catalin.marinas@....com>,
        "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
        Linuxarm <linuxarm@...wei.com>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Jonathan Cameron <jonathan.cameron@...wei.com>,
        Nicolas Saenz Julienne <nsaenzjulienne@...e.de>,
        Steve Capper <steve.capper@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Mike Rapoport <rppt@...ux.ibm.com>,
        "Zengtao (B)" <prime.zeng@...ilicon.com>,
        huangdaode <huangdaode@...wei.com>
Subject: Re: [PATCH v3 1/2] dma-direct: provide the ability to reserve
 per-numa CMA

On Wed, Jul 22, 2020 at 09:26:03PM +0000, Song Bao Hua (Barry Song) wrote:
> I understand your concern. Anyway, The primary purpose of this patchset is providing
> a general way for users like IOMMU to get local coherent dma buffers to put their
> command queue and page tables in. The first user case is what really made me
> begin to prepare this patchset.
> 
> For the second case, it is probably a positive side effect of this patchset for those users
> who have more concern on performance than dma security, then they maybe skip
> IOMMU by
> 	iommu.passthrough=
> 			[ARM64, X86] Configure DMA to bypass the IOMMU by default.
> 			Format: { "0" | "1" }
> 			0 - Use IOMMU translation for DMA.
> 			1 - Bypass the IOMMU for DMA.
> 			unset - Use value of CONFIG_IOMMU_DEFAULT_PASSTHROUGH.
> In this case, they can get local memory and get better performance.
> However, it is not the primary purpose of this patchset.

That's not what I mean.  Hardcoding the CMA regions in the kernel
config is just a bad idea, and we should not add more hard coded values.
You can always use CONFIG_CMDLINE to force a specific kernel command
line including your options.