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

[Xiao Guangrong]

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 <address@hidden> wrote:
> >
> > > On 02/08/2016 07:03 PM, Igor Mammedov wrote:
> > > > On Wed, 13 Jan 2016 02:50:05 +0800
> > > > Xiao Guangrong <address@hidden> 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.