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Subject: Re: [PATCH] iommu/amd: Enable swiotlb if any device supports iommu v2
 and uses identity mapping
To:     Joerg Roedel <joro@8bytes.org>,
        Kai-Heng Feng <kai.heng.feng@canonical.com>
Cc:     will@kernel.org,
        "open list:AMD IOMMU (AMD-VI)" <iommu@lists.linux-foundation.org>,
        open list <linux-kernel@vger.kernel.org>
References: <20210708074232.924844-1-kai.heng.feng@canonical.com>
 <YObFJREB9/JlcNZP@8bytes.org>
From:   Robin Murphy <robin.murphy@arm.com>
Message-ID: <fde11cec-d1bd-49be-f129-c69a973d1b3b@arm.com>
Date:   Thu, 8 Jul 2021 11:18:45 +0100
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On 2021-07-08 10:28, Joerg Roedel wrote:
> On Thu, Jul 08, 2021 at 03:42:32PM +0800, Kai-Heng Feng wrote:
>> @@ -344,6 +344,9 @@ static int iommu_init_device(struct device *dev)
>>   
>>   		iommu = amd_iommu_rlookup_table[dev_data->devid];
>>   		dev_data->iommu_v2 = iommu->is_iommu_v2;
>> +
>> +		if (dev_data->iommu_v2)
>> +			swiotlb = 1;
> 
> This looks like the big hammer, as it will affect all other systems
> where the AMD GPUs are in their own group.
> 
> What is needed here is an explicit check whether a non-iommu-v2 device
> is direct-mapped because it shares a group with the GPU, and only enable
> swiotlb in this case.

Right, it's basically about whether any DMA-limited device might at any 
time end up in an IOMMU_DOMAIN_IDENTITY domain. And given the 
possibility of device hotplug and the user being silly with the sysfs 
interface, I don't think we can categorically determine that at boot time.

Also note that Intel systems are likely to be similarly affected (in 
fact intel-iommu doesn't even have the iommu_default_passthough() check 
so it's probably even easier to blow up).

Robin.