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

From: Robin Murphy
Date: Thu Jul 08 2021 - 06:18:53 EST

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).