linux-kernel - Re: [PATCH v3 02/10] swiotlb: Factor out slot allocation and free

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Date:   Mon, 29 Apr 2019 13:10:17 +0800
From:   Lu Baolu <baolu.lu@...ux.intel.com>
To:     Christoph Hellwig <hch@....de>
Cc:     baolu.lu@...ux.intel.com, David Woodhouse <dwmw2@...radead.org>,
        Joerg Roedel <joro@...tes.org>, ashok.raj@...el.com,
        jacob.jun.pan@...el.com, alan.cox@...el.com, kevin.tian@...el.com,
        mika.westerberg@...ux.intel.com, pengfei.xu@...el.com,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Robin Murphy <robin.murphy@....com>,
        iommu@...ts.linux-foundation.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 02/10] swiotlb: Factor out slot allocation and free

Hi Christoph,

On 4/26/19 11:04 PM, Christoph Hellwig wrote:
> On Thu, Apr 25, 2019 at 10:07:19AM +0800, Lu Baolu wrote:
>> This is not VT-d specific. It's just how generic IOMMU works.
>>
>> Normally, IOMMU works in paging mode. So if a driver issues DMA with
>> IOVA  0xAAAA0123, IOMMU can remap it with a physical address 0xBBBB0123.
>> But we should never expect IOMMU to remap 0xAAAA0123 with physical
>> address of 0xBBBB0000. That's the reason why I said that IOMMU will not
>> work there.
> 
> Well, with the iommu it doesn't happen.  With swiotlb it obviosuly
> can happen, so drivers are fine with it.  Why would that suddenly
> become an issue when swiotlb is called from the iommu code?
> 

I would say IOMMU is DMA remapping, not DMA engine. :-)

Best regards,
Lu Baolu