Re: [Qemu-devel] [PATCH v2 06/11] nvdimm acpi: initialize the resource used by NVDIMM ACPI

[Xiao Guangrong <guangrong.xiao@linux.intel.com>]

On 02/15/2016 09:32 PM, Igor Mammedov wrote:
> On Mon, 15 Feb 2016 13:45:59 +0200
> "Michael S. Tsirkin" <mst@redhat.com> wrote:
>
>> On Mon, Feb 15, 2016 at 11:47:42AM +0100, Igor Mammedov wrote:
>>> On Mon, 15 Feb 2016 18:13:38 +0800
>>> Xiao Guangrong <guangrong.xiao@linux.intel.com> wrote:
>>>
>>>> On 02/15/2016 05:18 PM, Michael S. Tsirkin wrote:
>>>>> On Mon, Feb 15, 2016 at 10:11:05AM +0100, Igor Mammedov wrote:
>>>>>> On Sun, 14 Feb 2016 13:57:27 +0800
>>>>>> Xiao Guangrong <guangrong.xiao@linux.intel.com> wrote:
>>>>>>
>>>>>>> On 02/08/2016 07:03 PM, Igor Mammedov wrote:
>>>>>>>> On Wed, 13 Jan 2016 02:50:05 +0800
>>>>>>>> Xiao Guangrong <guangrong.xiao@linux.intel.com> wrote:
>>>>>>>>
>>>>>>>>> 32 bits IO port starting from 0x0a18 in guest is reserved for NVDIMM
>>>>>>>>> ACPI emulation. The table, NVDIMM_DSM_MEM_FILE, will be patched into
>>>>>>>>> NVDIMM ACPI binary code
>>>>>>>>>
>>>>>>>>> OSPM uses this port to tell QEMU the final address of the DSM memory
>>>>>>>>> and notify QEMU to emulate the DSM method
>>>>>>>> Would you need to pass control to QEMU if each NVDIMM had its whole
>>>>>>>> label area MemoryRegion mapped right after its storage MemoryRegion?
>>>>>>>>
>>>>>>>
>>>>>>> No, label data is not mapped into guest's address space and it only
>>>>>>> can be accessed by DSM method indirectly.
>>>>>> Yep, per spec label data should be accessed via _DSM but question
>>>>>> wasn't about it,
>>>>
>>>> Ah, sorry, i missed your question.
>>>>
>>>>>> Why would one map only 4Kb window and serialize label data
>>>>>> via it if it could be mapped as whole, that way _DMS method will be
>>>>>> much less complicated and there won't be need to add/support a protocol
>>>>>> for its serialization.
>>>>>>
>>>>>
>>>>> Is it ever accessed on data path? If not I prefer the current approach:
>>>>
>>>> The label data is only accessed via two DSM commands - Get Namespace Label
>>>> Data and Set Namespace Label Data, no other place need to be emulated.
>>>>
>>>>> limit the window used, the serialization protocol seems rather simple.
>>>>>
>>>>
>>>> Yes.
>>>>
>>>> Label data is at least 128k which is big enough for BIOS as it allocates
>>>> memory at 0 ~ 4G which is tight region. It also needs guest OS to support
>>>> lager max-xfer (the max size that can be transferred one time), the size
>>>> in current Linux NVDIMM driver is 4k.
>>>>
>>>> However, using lager DSM buffer can help us to simplify NVDIMM hotplug for
>>>> the case that too many nvdimm devices present in the system and their FIT
>>>> info can not be filled into one page. Each PMEM-only device needs 0xb8 bytes
>>>> and we can append 256 memory devices at most, so 12 pages are needed to
>>>> contain this info. The prototype we implemented is using ourself-defined
>>>> protocol to read piece of _FIT and concatenate them before return to Guest,
>>>> please refer to:
>>>> https://github.com/xiaogr/qemu/commit/c46ce01c8433ac0870670304360b3c4aa414143a
>>>>
>>>> As 12 pages are not small region for BIOS and the _FIT size may be extended in the
>>>> future development (eg, if PBLK is introduced) i am not sure if we need this. Of
>>>> course, another approach to simplify it is that we limit the number of NVDIMM
>>>> device to make sure their _FIT < 4k.
>>> My suggestion is not to have only one label area for every NVDIMM but
>>> rather to map each label area right after each NVDIMM's data memory.
>>> That way _DMS can be made non-serialized and guest could handle
>>> label data in parallel.
>>
>> I think that alignment considerations would mean we are burning up
>> 1G of phys address space for this. For PAE we only have 64G
>> of this address space, so this would be a problem.
> That's true that it will burning away address space, however that
> just means that PAE guests would not be able to handle as many
> NVDIMMs as 64bit guests. The same applies to DIMMs as well, with
> alignment enforced. If one needs more DIMMs he/she can switch
> to 64bit guest to use them.
>
> It's trade of inefficient GPA consumption vs efficient NVDIMMs access.
> Also with fully mapped label area for each NVDIMM we don't have to
> introduce and maintain any guest visible serialization protocol
> (protocol for serializing _DSM via 4K window) which becomes ABI.

It's true for label access but it is not for the long term as we will
need to support other _DSM commands such as vendor specific command,
PBLK dsm command, also NVDIMM MCE related commands will be introduced
in the future, so we will come back here at that time. :(