> From: Richard Yao [mailto:[email protected]] > Sent: Tuesday, November 12, 2013 9:00 AM > > I recompiled my kernel with support and I now see some rather reassuring > messages in dmesg about an IOMMU being used for various devices. It is
To clarify, do you mean CONFIG_INTEL_IOMMU and CONFIG_INTEL_IOMMU_DEFAULT_ON? It would appear the former is generally configured on the average distribution, but not the latter, requiring an explicit kernel parameter to be supplied at boot in order to enable it. > not clear to me that Linux distributions ship with this enabled, > although that is a concern for ZFSOnLinux and various Linux > distributions. I will patch Gentoo's genkernel software to add support > for this functionality by default. So you are going to enable CONFIG_INTEL_IOMMU_DEFAULT_ON in the default Gentoo kernel config from now on? There was a mention of a similar feature in FreeBSD, I'm not sure if it changed since this posting discussing it: http://lists.freebsd.org/pipermail/freebsd-arch/2013-May/014368.html " It is known that IOMMU adds overhead due to the mapping and unmapping for each I/O. DMAR implementations usually have some erratas, as well as PCIe devices sometime do not completely follow the specification, causing misbehaviour with remapping enabled. For this reason I do not plan to enable IOMMU by default" but at the time it sounds like they weren't going to enable it by default, which would make it similar to the current average linux kernel, which supports it, but doesn't enable it by default? Any idea what the performance impact of the overhead is? Or how many DMAR implementations are buggy :)? _______________________________________________ developer mailing list [email protected] http://lists.open-zfs.org/mailman/listinfo/developer
