linux-kernel - Re: [PATCH v5 02/10] iommu/vt-d: Use per-device dma

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Message-ID: <20191113070312.GA2735@lst.de>
Date:   Wed, 13 Nov 2019 08:03:12 +0100
From:   Christoph Hellwig <hch@....de>
To:     Lu Baolu <baolu.lu@...ux.intel.com>
Cc:     Christoph Hellwig <hch@....de>,
        David Woodhouse <dwmw2@...radead.org>,
        Joerg Roedel <joro@...tes.org>,
        Bjorn Helgaas <bhelgaas@...gle.com>, ashok.raj@...el.com,
        jacob.jun.pan@...el.com, alan.cox@...el.com, kevin.tian@...el.com,
        mika.westerberg@...ux.intel.com, Ingo Molnar <mingo@...hat.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        pengfei.xu@...el.com,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Robin Murphy <robin.murphy@....com>,
        Jonathan Corbet <corbet@....net>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Juergen Gross <jgross@...e.com>,
        Stefano Stabellini <sstabellini@...nel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        iommu@...ts.linux-foundation.org, linux-kernel@...r.kernel.org,
        Jacob Pan <jacob.jun.pan@...ux.intel.com>
Subject: Re: [PATCH v5 02/10] iommu/vt-d: Use per-device dma_ops

On Wed, Nov 13, 2019 at 10:50:27AM +0800, Lu Baolu wrote:
> Currently, this is a block issue for using per-device dma ops in Intel
> IOMMU driver. Hence block this driver from using the generic iommu dma
> ops.

That is in fact the reason why I bring it up :)

> I'd like to align Intel IOMMU driver with other vendors. Use iommu dma
> ops for devices which have been selected to go through iommu. And use
> direct dma ops if selected to by pass.
>
> One concern of this propose is that for devices with limited address
> capability, shall we force it to use iommu or alternatively use swiotlb
> if user decides to let it by pass iommu.
>
> I understand that using swiotlb will cause some overhead due to the
> bounced buffer, but Intel IOMMU is default on hence any users who use a
> default kernel won't suffer this. We only need to document this so that
> users understand this overhead when they decide to let such devices by
> pass iommu. This is common to all vendor iommu drivers as far as I can
> see.

Indeed.  And one idea would be to lift the code in the powerpc
dma_iommu_ops that check a flag and use the direct ops to the generic
dma code and a flag in struct device.  We can then switch the intel
iommu ops (and AMD Gart) over to it.