On Wed, 22 Mar 2017 11:06:47 +0000
Gabor Locsei <gabor.loc...@ericsson.com> wrote:

> Hello,
> 
> I have systems with IOMMU optionally enabled, and I would like to find a 
> method to check these capabilities that doesn't rely on dmesg.
> The evident problem is that dmesg eventually clear boot time messages so the 
> relevant lines may just disappear.
> 
> Looks like there are sysfs entries that seem to be fit for this purpose. 
> 
> DMAR                  The file /sys/firmware/acpi/tables/DMAR exists. (This 
> file exists on every lab node.)
> IOMMU                 The directory /sys/devices/virtual/iommu/ exists and 
> there are dmar0, etc device subdirectories in it
> 
> I have looked into the ACPI DMAR tables with iasl, but they look the same 
> regardless of the cmdline iommu settings.
> The question is, provided that the platform has ACPI, is it safe to rely on 
> sysfs, is the above enough to prove that IOMMU is available? The oldest 
> kernel version to verify against is 3.10.0 from RHEL.

DMAR is a firmware table, it's present so long as VT-d is enabled in
the BIOS, irrespective of whether the IOMMU is enabled in the kernel.
The IOMMU sysfs support was added exactly for this purpose (I tend to
prefer reference via /sys/class/iommu).

IOMMU sysfs support was added in v3.17, but note that the RHEL version
of v3.10 is a moving target, not a fixed point in time.  This support
was added in the RHEL 7.1 kernel.  Thanks,

Alex
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