I'd like to verify on any given Linux machine if PCI passthrough is supported. After a bit of googling, I found that I should rather check if IOMMU is supported, and I did so by running:

dmesg | grep IOMMU 

If it supports IOMMU (and not IOMMUv2), I would get:

IOMMU
[ 0.000000] DMAR: IOMMU enabled
[ 0.049734] DMAR-IR: IOAPIC id 8 under DRHD base 0xfbffc000 IOMMU 0
[ 0.049735] DMAR-IR: IOAPIC id 9 under DRHD base 0xfbffc000 IOMMU 0
[ 1.286567] AMD IOMMUv2 driver by Joerg Roedel <>
[ 1.286568] AMD IOMMUv2 functionality not available on this system

...where DMAR: IOMMU enabled is what I'm looking for.

Now, if the machine has been running for days without a reboot, that first message [ 0.000000] DMAR: IOMMU enabled might not appear any more in the log with the previous command.

Is there any way to check for IOMMU support when that message disappears from the log?

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1 Answer

Since 2014 enabled iommu are registered in /sys (sysfs) special file system as class iommu (documented at ABI/testing/sysfs-class-iommu): "[2/3] iommu/intel: Make use of IOMMU sysfs support" - June 12, 2014

Register our DRHD IOMMUs, cross link devices, and provide a base set of attributes for the IOMMU. ... On a typical desktop system, this provides the following (pruned):

$ find /sys | grep dmar
/sys/devices/virtual/iommu/dmar0
...
/sys/class/iommu/dmar0
/sys/class/iommu/dmar1

The code is iommu_device_create (, around 4.5) or iommu_device_sysfs_add () in more recent kernels.

/* * Create an IOMMU device and return a pointer to it. IOMMU specific * attributes can be provided as an attribute group, allowing a unique * namespace per IOMMU type. */
struct device *iommu_device_create(struct device *parent, void *drvdata, const struct attribute_group **groups, const char *fmt, ...)

Registration is done only for enabled IOMMU. DMAR:

if (intel_iommu_enabled) { iommu->iommu_dev = iommu_device_create(NULL, iommu, intel_iommu_groups, "%s", iommu->name);

AMD IOMMU:

static int iommu_init_pci(struct amd_iommu *iommu)
{ ... if (!iommu->dev) return -ENODEV;
... iommu->iommu_dev = iommu_device_create(&iommu->dev->dev, iommu, amd_iommu_groups, "ivhd%d", iommu->index);

Intel:

int __init intel_iommu_init(void)
{ ... pr_info("Intel(R) Virtualization Technology for Directed I/O\n");
... for_each_active_iommu(iommu, drhd) iommu->iommu_dev = iommu_device_create(NULL, iommu, intel_iommu_groups, "%s", iommu->name);

With 4.11 linux kernel version iommu_device_sysfs_add is referenced in many IOMMU drivers, so checking /sys/class/iommu is better (more universal) way to programmatically detect enabled IOMMU than parsing dmesg output or searching in /var/log/kern.log or /var/log/messages for driver-specific enable messages:

Referenced in 10 files:

• drivers/iommu/amd_iommu_init.c, line 1640
• drivers/iommu/arm-smmu-v3.c, line 2709
• drivers/iommu/arm-smmu.c, line 2163
• drivers/iommu/dmar.c, line 1083
• drivers/iommu/exynos-iommu.c, line 623
• drivers/iommu/intel-iommu.c, line 4878
• drivers/iommu/iommu-sysfs.c, line 57
• drivers/iommu/msm_iommu.c, line 797
• drivers/iommu/mtk_iommu.c, line 581

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