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Diffstat (limited to 'docs')
| -rw-r--r-- | docs/nvdimm.txt | 31 |
1 files changed, 31 insertions, 0 deletions
diff --git a/docs/nvdimm.txt b/docs/nvdimm.txt index 2d9f8c0e8c..e903d8bb09 100644 --- a/docs/nvdimm.txt +++ b/docs/nvdimm.txt @@ -122,3 +122,34 @@ Note: M >= size of RAM devices + size of statically plugged vNVDIMM devices + size of hotplugged vNVDIMM devices + +Alignment +--------- + +QEMU uses mmap(2) to maps vNVDIMM backends and aligns the mapping +address to the page size (getpagesize(2)) by default. However, some +types of backends may require an alignment different than the page +size. In that case, QEMU v2.12.0 and later provide 'align' option to +memory-backend-file to allow users to specify the proper alignment. + +For example, device dax require the 2 MB alignment, so we can use +following QEMU command line options to use it (/dev/dax0.0) as the +backend of vNVDIMM: + + -object memory-backend-file,id=mem1,share=on,mem-path=/dev/dax0.0,size=4G,align=2M + -device nvdimm,id=nvdimm1,memdev=mem1 + +Guest Data Persistence +---------------------- + +Though QEMU supports multiple types of vNVDIMM backends on Linux, +currently the only one that can guarantee the guest write persistence +is the device DAX on the real NVDIMM device (e.g., /dev/dax0.0), to +which all guest access do not involve any host-side kernel cache. + +When using other types of backends, it's suggested to set 'unarmed' +option of '-device nvdimm' to 'on', which sets the unarmed flag of the +guest NVDIMM region mapping structure. This unarmed flag indicates +guest software that this vNVDIMM device contains a region that cannot +accept persistent writes. In result, for example, the guest Linux +NVDIMM driver, marks such vNVDIMM device as read-only. |