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

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Date:   Thu, 25 Jul 2019 15:18:03 +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>,
        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

Hi Christoph,

On 7/25/19 1:44 PM, Christoph Hellwig wrote:
>>   /* Check if the dev needs to go through non-identity map and unmap process.*/
>>   static bool iommu_need_mapping(struct device *dev)
>>   {
>> -	int ret;
>> -
>>   	if (iommu_dummy(dev))
>>   		return false;
>>   
>> -	ret = identity_mapping(dev);
>> -	if (ret) {
>> -		u64 dma_mask = *dev->dma_mask;
>> -
>> -		if (dev->coherent_dma_mask && dev->coherent_dma_mask < dma_mask)
>> -			dma_mask = dev->coherent_dma_mask;
>> -
>> -		if (dma_mask >= dma_get_required_mask(dev))
>> -			return false;
> 
> Don't we need to keep this bit so that we still allow the IOMMU
> to act if the device has a too small DMA mask to address all memory in
> the system, even if if it should otherwise be identity mapped?
> 

This checking happens only when device is using an identity mapped
domain. If the device has a small DMA mask, swiotlb will be used for
high memory access.

This is supposed to be handled in dma_direct_map_page():

         if (unlikely(!dma_direct_possible(dev, dma_addr, size)) &&
             !swiotlb_map(dev, &phys, &dma_addr, size, dir, attrs)) {
                 report_addr(dev, dma_addr, size);
                 return DMA_MAPPING_ERROR;
         }

Best regards,
Baolu