IOTLB page size question

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.

Quoted 1 line · Jan Sacha

We have tried two different Linux distributions (CentOS and Fedora)...

This doesn't really help narrow your kernel version.

0xe0000 pages? 0xe000 pages isn't 3.75G.

Legacy KVM device assignment maps IOMMU pages using the host kernel page size for the region while VFIO will pass the largest contiguous range of pages available to the IOMMU, regardless of kernel page size. If VFIO doesn't have the same problem then perhaps the kernel idea of the page size for that reason has changed between mappings. Thanks,

Quoted 5 lines

Legacy KVM device assignment maps IOMMU pages using the host kernel page size for the region while VFIO will pass the largest contiguous range of pages available to the IOMMU, regardless of kernel page size. If VFIO doesn't have the same problem then perhaps the kernel idea of the page size for that reason has changed between mappings. Thanks,

When I have a VM running, I can see on the host OS:

# cat /proc/1360/numa_maps
2aaaaac00000 prefer:0 file=/dev/hugepages/libvirt/qemu/qemu_back_mem.DO2GNu\040(deleted) huge dirty=3070 N0=3070
...

# cat /proc/1360/maps
2aaaaac00000-2aac2a800000 rw-s 00000000 00:1e 19928 /dev/hugepages/libvirt/qemu/qemu_back_mem.DO2GNu (deleted)
...

So it looks to me that there should be roughly 6GB mapped using huge (2M) pages. PID 1360 is the qemu process. The VM is configured to use 6GB. However, the IOTLB seems to be using 2k pages for all memory below 4GB in the guest physical space. It does use 2M pages for memory above 4GB. Does this make sense?