Re: [Qemu-devel] QEMU NVDIMM as type 7 in e820 table

[Ross Zwisler <ross.zwisler@linux.intel.com>]

On Sat, Jul 29, 2017 at 06:49:33PM +0800, Haozhong Zhang wrote:
> On 07/28/17 13:45 -0600, Ross Zwisler wrote:
> > On Fri, Jul 28, 2017 at 11:11:10AM -0700, Dan Williams wrote:
> > > On Fri, Jul 28, 2017 at 11:04 AM, Ross Zwisler
> > > <ross.zwisler@linux.intel.com> wrote:
> > > > I've been using the virtualized NVDIMM support in QEMU for testing, and I
> > > > noticed that the physical addresses used by the virtual NVDIMMs aren't present
> > > > in the guest's e820 table.
> > > >
> > > > Here is the e820 table on my QEMU instance where I have one 32 GiB virtual
> > > > NVDIMM:
> > > >
> > > > [    0.000000] e820: BIOS-provided physical RAM map:
> > > > [    0.000000] BIOS-e820: [mem 0x0000000000000000-0x000000000009fbff] usable
> > > > [    0.000000] BIOS-e820: [mem 0x000000000009fc00-0x000000000009ffff] reserved
> > > > [    0.000000] BIOS-e820: [mem 0x00000000000f0000-0x00000000000fffff] reserved
> > > > [    0.000000] BIOS-e820: [mem 0x0000000000100000-0x00000000bffdefff] usable
> > > > [    0.000000] BIOS-e820: [mem 0x00000000bffdf000-0x00000000bfffffff] reserved
> > > > [    0.000000] BIOS-e820: [mem 0x00000000feffc000-0x00000000feffffff] reserved
> > > > [    0.000000] BIOS-e820: [mem 0x00000000fffc0000-0x00000000ffffffff] reserved
> > > > [    0.000000] BIOS-e820: [mem 0x0000000100000000-0x000000023fffffff] usable
> > > >
> > > > The physical addresses used by the virtual NVDIMM are 0x240000000-0xA40000000.
> > > > You can see this by looking at ndctl and the values we get from the NFIT:
> > > >
> > > > # ndctl list -R
> > > > {
> > > >   "dev":"region0",
> > > >   "size":34359738368,
> > > >   "available_size":0,
> > > >   "type":"pmem"
> > > > }
> > > >
> > > > # grep . /sys/bus/nd/devices/region0/{resource,size}
> > > > region0/resource:0x240000000
> > > > region0/size:34359738368
> > > >
> > > > Or you can see the same info by using iasl to dump
> > > > /sys/firmware/acpi/tables/NFIT:
> > > >
> > > > [028h 0040   2]                Subtable Type : 0000 [System Physical Address Range]
> > > > [02Ah 0042   2]                       Length : 0038
> > > >
> > > > [02Ch 0044   2]                  Range Index : 0002
> > > > [02Eh 0046   2]        Flags (decoded below) : 0003
> > > >                    Add/Online Operation Only : 1
> > > >                       Proximity Domain Valid : 1
> > > > [030h 0048   4]                     Reserved : 00000000
> > > > [034h 0052   4]             Proximity Domain : 00000000
> > > > [038h 0056  16]           Address Range GUID : 66F0D379-B4F3-4074-AC43-0D3318B78CDB
> > > > [048h 0072   8]           Address Range Base : 0000000240000000
> > > > [050h 0080   8]         Address Range Length : 0000000800000000
> > > > [058h 0088   8]         Memory Map Attribute : 0000000000008008
> > > >
> > > > I expected to see a type 7 region for the NVDIMM physical address range in the
> > > > e820 table, so something like:
> > > >
> > > > [    0.000000] e820: BIOS-provided physical RAM map:
> > > > [    0.000000] BIOS-e820: [mem 0x0000000000000000-0x000000000009fbff] usable
> > > > [    0.000000] BIOS-e820: [mem 0x000000000009fc00-0x000000000009ffff] reserved
> > > > [    0.000000] BIOS-e820: [mem 0x00000000000f0000-0x00000000000fffff] reserved
> > > > [    0.000000] BIOS-e820: [mem 0x0000000000100000-0x00000000bffdefff] usable
> > > > [    0.000000] BIOS-e820: [mem 0x00000000bffdf000-0x00000000bfffffff] reserved
> > > > [    0.000000] BIOS-e820: [mem 0x00000000feffc000-0x00000000feffffff] reserved
> > > > [    0.000000] BIOS-e820: [mem 0x00000000fffc0000-0x00000000ffffffff] reserved
> > > > [    0.000000] BIOS-e820: [mem 0x0000000100000000-0x000000023fffffff] usable
> > > > [    0.000000] BIOS-e820: [mem 0x0000000240000000-0x0000000A40000000] persistent (type 7)
> > > >
> > > 
> > > Do you need that informationin e820? Linux effectively ignores type-7.
> > > As long as the range is treated as reserved it's not clear that you
> > > need the e820 entry. We also infect the persistent type back into the
> > > memory map when the NFIT driver loads. /proc/iomem should show the
> > > right data.
> > 
> > [ Adding Linda & Toshi to see if they have an opinion. ]
> > 
> > I guess maybe we don't need it.  Yep, /proc/iomem looks good:
> > 
> >   # cat /proc/iomem
> >   00000000-00000fff : Reserved
> >   00001000-0009fbff : System RAM
> >   ...
> >   100000000-23fffffff : System RAM
> >   240000000-a3fffffff : Persistent Memory
> >     240000000-a3fffffff : namespace0.0
> > 
> > I was just worried that this was an inconsistency between the way that virtual
> > NVDIMMs are presented vs the way that they will be presented on bare metal.  I
> > at least look at the e820 table to get my bearings of how memory is laid out -
> > maybe I just need to look at /proc/iomem instead?
> 
> Do any OS or applications rely on the E820 information or the
> consistency between E820 and NFIT to properly work? If any, I can make
> a QEMU patch to build type-7 e820 entries.

I don't know of any off hand, but IMO it would be good to have this
consistency.