[libvirt] [PATCH 0/5] Introduce NVDIMM support

Michal Privoznik mprivozn at redhat.com
Thu Aug 11 08:12:04 UTC 2016

On 04.08.2016 10:42, Stefan Hajnoczi wrote:
> On Mon, Aug 1, 2016 at 4:10 PM, Michal Privoznik <mprivozn at redhat.com> wrote:
>> NVDIMM was introduced to qemu in v2.6.0-rc0~248^2~25. So it's
>> been a while since then.
>> It's not the next big thing, but it is very interesting feature
>> enabling higher performance as reading/writing to the module (and
>> subsequently to the file on the host) does not require a VMEXIT.
>> It can be used to access host files directly bypassing page cache
>> whilst doing so.
>> How to test the feature?
>> 1) you need PMEM enabled kernel:
>> 2) Create a file in the host:
>>   truncate -s 512M /tmp/nvdimm
>> 3) Add the following to the domain XML:
>>     <memory model='nvdimm' memAccess='shared'>
>>       <source>
>>         <path>/tmp/nvdimm</path>
>>       </source>
>>       <target>
>>         <size unit='KiB'>523264</size>
>>         <node>0</node>
>>       </target>
>>     </memory>
> The "nvdimm" device also has a label-size property.  This determines
> the size of the Namespace Label area described in:
> http://pmem.io/documents/NVDIMM_Namespace_Spec.pdf
> By default no Namespace Label area is reserved in the file.  If the
> user specifies label-size then the memory at the end of the file is
> used as the Namespace Label area.
> It is necessary to expose label-size so users can choose whether or
> not to have a Namespace Label area.
> I have CCed Guangrong Xiao who authored the QEMU patches.

Ah, thank you for that. From the code I understand that the minimum
value for that is 128K, but what about other restrictions? Does the
number need to be aligned? What is the minimal step between two
different values?


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