linux-kernel - Re: [PATCH] iommu/amd: Enable swiotlb if any device supports iommu v2 and uses identity mapping

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Date:   Thu, 8 Jul 2021 11:28:05 +0200
From:   Joerg Roedel <joro@...tes.org>
To:     Kai-Heng Feng <kai.heng.feng@...onical.com>
Cc:     will@...nel.org, Robin Murphy <robin.murphy@....com>,
        "open list:AMD IOMMU (AMD-VI)" <iommu@...ts.linux-foundation.org>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] iommu/amd: Enable swiotlb if any device supports iommu
 v2 and uses identity mapping

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.

Thanks,

	Joerg