Hi,
I have a question about IOTLB. We are running KVM/Qemu VMs with huge
page memory backing on Intel Xeon Ivy Bridge machines. Our VMs use 10G
Ether NICs in Intel VT-d mode. We actually see that IOTLB becomes a
performance bottleneck when IOMMU uses 4k pages. We get much better
packet throughput with 2M IOTLB pages.
We have tried two different Linux distributions (CentOS and Fedora). An
older CentOS kernel maps everything using 4k IOTLB pages. Our newer
Fedora kernel initially maps guest memory using 2M IOTLB pages, but we
see that a couple of seconds later it remaps the first 0xE000 pages
(3.75GB) of memory using 4k IOTLB pages. We still have 2M IOTLB page
mappings for memory above 4GB.
Why would the kernel change the IOTLB page size from 2M to 4k? How can
we make sure that all memory (except for some non-aligned bits) gets
mapped using 2M IOTLB pages? As I mentioned, we are using huge-page
memory backing for all our VMs. Any advice, also for debugging and
further diagnosis, would be appreciated.
Jan
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