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Date:   Thu, 20 Feb 2020 09:24:41 -0700
From:   Jerry Snitselaar <jsnitsel@redhat.com>
To:     Lu Baolu <baolu.lu@linux.intel.com>
Cc:     Joerg Roedel <joro@8bytes.org>, iommu@lists.linux-foundation.org,
        linux-kernel@vger.kernel.org
Subject: Re: question about iommu_need_mapping
Message-ID: <20200220162441.bhnpwgsmj4vlp3ve@cantor>
Reply-To: Jerry Snitselaar <jsnitsel@redhat.com>
Mail-Followup-To: Lu Baolu <baolu.lu@linux.intel.com>,
        Joerg Roedel <joro@8bytes.org>, iommu@lists.linux-foundation.org,
        linux-kernel@vger.kernel.org
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On Thu Feb 20 20, Lu Baolu wrote:
>Hi Jerry,
>
>On 2020/2/20 7:55, Jerry Snitselaar wrote:
>>Is it possible for a device to end up with dev->archdata.iommu == NULL
>>on iommu_need_mapping in the following instance:
>>
>>1. iommu_group has dma domain for default
>>2. device gets private identity domain in intel_iommu_add_device
>>3. iommu_need_mapping gets called with that device.
>>4. dmar_remove_one_dev_info sets dev->archdata.iommu = NULL via 
>>unlink_domain_info.
>>5. request_default_domain_for_dev exits after checking that 
>>group->default_domain
>>    exists, and group->default_domain->type is dma.
>>6. iommu_request_dma_domain_for_dev returns 0 from 
>>request_default_domain_for_dev
>>    and a private dma domain isn't created for the device.
>>
>
>Yes. It's possible.
>
>>The case I was seeing went away with commit 9235cb13d7d1 ("iommu/vt-d:
>>Allow devices with RMRRs to use identity domain"), because it changed
>>which domain the group and devices were using, but it seems like it is
>>still a possibility with the code. Baolu, you mentioned possibly
>>removing the domain switch. Commit 98b2fffb5e27 ("iommu/vt-d: Handle
>>32bit device with identity default domain") makes it sound like the
>>domain switch is required.
>
>It's more "nice to have" than "required" if the iommu driver doesn't
>disable swiotlb explicitly. The device access of system memory higher
>than the device's addressing capability could go through the bounced
>buffer implemented in swiotlb.
>
>Best regards,
>baolu

Hi Baolu,

Would this mean switching to bounce_dma_ops instead?

Regards,
Jerry

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