* Haozhong Zhang (haozhong.zh...@intel.com) wrote: > Signed-off-by: Haozhong Zhang <haozhong.zh...@intel.com> > Reviewed-by: Xiao Guangrong <guangrong.x...@linux.intel.com> > --- > docs/nvdimm.txt | 124 > ++++++++++++++++++++++++++++++++++++++++++++++++++++++++ > 1 file changed, 124 insertions(+) > create mode 100644 docs/nvdimm.txt > > diff --git a/docs/nvdimm.txt b/docs/nvdimm.txt > new file mode 100644 > index 0000000..fafca39 > --- /dev/null > +++ b/docs/nvdimm.txt > @@ -0,0 +1,124 @@ > +QEMU Virtual NVDIMM > +=================== > + > +This document explains the usage of virtual NVDIMM (vNVDIMM) feature > +which is available since QEMU v2.6.0. > + > +The current QEMU only implements the persistent memory mode of vNVDIMM > +device. > + > +Basic Usage > +----------- > + > +The storage of a vNVDIMM device in QEMU is provided by the memory > +backend (i.e. memory-backend-file and memory-backend-ram). A simple > +way to create a vNVDIMM device at startup time is done via the > +following command line options: > + > + -machine pc,nvdimm > + -m $RAM_SIZE,slots=$N,maxmem=$MAX_SIZE > + -object > memory-backend-file,id=mem1,share=on,mem-path=$PATH,size=$NVDIMM_SIZE > + -device nvdimm,id=nvdimm1,memdev=mem1 > + > +Where, > + > + - the "nvdimm" machine option enables vNVDIMM feature. > + > + - "slots=$N" should be equal to or larger than the total amount of > + normal RAM devices and vNVDIMM devices, e.g. $N should be >= 2 here. > + > + - "maxmem=$MAX_SIZE" should be equal to or larger than the total size > + of normal RAM devices and vNVDIMM devices, e.g. $MAX_SIZE should be > + >= $RAM_SIZE + $NVDIMM_SIZE here. > + > + - "object > memory-backend-file,id=mem1,share=on,mem-path=$PATH,size=$NVDIMM_SIZE" > + creates a backend storage of size $NVDIMM_SIZE on a file $PATH. All > + accesses to the virtual NVDIMM device go to the file $PATH. > + > + "share=on/off" controls the visibility of guest writes. If > + "share=on", then guest writes will be applied to the backend > + file. If another guest uses the same backend file with option > + "share=on", then above writes will be visible to it as well. If > + "share=off", then guest writes won't be applied to the backend > + file and thus will be invisible to other guests. > + > + - "device nvdimm,id=nvdimm1,memdev=mem1" creates a virtual NVDIMM > + device whose storage is provided by above memory backend device. > + > +Multiple vNVDIMM devices can be created if multiple pairs of "-object" > +and "-device" are provided. > + > +For above command line options, if the guest OS has the proper NVDIMM > +driver, it should be able to detect a NVDIMM device which is in the > +persistent memory mode and whose size is $NVDIMM_SIZE. > + > +Note: > + > +1. Prior to QEMU v2.8.0, if memory-backend-file is used and the actual > + backend file size is not equal to the size given by "size" option, > + QEMU will truncate the backend file by ftruncate(2), which will > + corrupt the existing data in the backend file, especially for the > + shrink case. > + > + QEMU v2.8.0 and later check the backend file size and the "size" > + option. If they do not match, QEMU will report errors and abort in > + order to avoid the data corruption. > + > +2. QEMU v2.6.0 only puts a basic alignment requirement on the "size" > + option of memory-backend-file, e.g. 4KB alignment on x86. However, > + QEMU v.2.7.0 puts an additional alignment requirement, which may > + require a larger value than the basic one, e.g. 2MB on x86. This > + change breaks the usage of memory-backend-file that only satisfies > + the basic alignment. > + > + QEMU v2.8.0 and later remove the additional alignment on non-s390x > + architectures, so the broken memory-backend-file can work again. > + > +Label > +----- > + > +QEMU v2.7.0 and later implement the label support for vNVDIMM devices. > +To enable label on vNVDIMM devices, users can simply add > +"label-size=$SZ" option to "-device nvdimm", e.g. > + > + -device nvdimm,id=nvdimm1,memdev=mem1,label_size=128K
Is that label-size rather than label_size ? Dave > + > +Note: > + > +1. The minimal label size is 128KB. > + > +2. QEMU v2.7.0 and later store labels at the end of backend storage. > + If a memory backend file, which was previously used as the backend > + of a vNVDIMM device without labels, is now used for a vNVDIMM > + device with label, the data in the label area at the end of file > + will be inaccessible to the guest. If any useful data (e.g. the > + meta-data of the file system) was stored there, the latter usage > + may result guest data corruption (e.g. breakage of guest file > + system). > + > +Hotplug > +------- > + > +QEMU v2.8.0 and later implement the hotplug support for vNVDIMM > +devices. Similarly to the RAM hotplug, the vNVDIMM hotplug is > +accomplished by two monitor commands "object_add" and "device_add". > + > +For example, the following commands add another 4GB vNVDIMM device to > +the guest: > + > + (qemu) object_add > memory-backend-file,id=mem2,share=on,mem-path=new_nvdimm.img,size=4G > + (qemu) device_add nvdimm,id=nvdimm2,memdev=mem2 > + > +Note: > + > +1. Each hotplugged vNVDIMM device consumes one memory slot. Users > + should always ensure the memory option "-m ...,slots=N" specifies > + enough number of slots, i.e. > + N >= number of RAM devices + > + number of statically plugged vNVDIMM devices + > + number of hotplugged vNVDIMM devices > + > +2. The similar is required for the memory option "-m ...,maxmem=M", i.e. > + M >= size of RAM devices + > + size of statically plugged vNVDIMM devices + > + size of hotplugged vNVDIMM devices > -- > 2.10.1 > > -- Dr. David Alan Gilbert / dgilb...@redhat.com / Manchester, UK
