Re: IOTLB page size question

[Jan Sacha <Jan.Sacha@alcatel-lucent.com>]

On 10/13/2014 04:16 PM, Jan Sacha wrote:
>> 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?

We found a solution so I can answer my own question. This behavior was 
caused by a bug in the kernel. It was fixed in 3.13.

https://github.com/torvalds/linux/commit/e0230e1327fb862c9b6cde24ae62d55f9db62c9b

Jan