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Re: [Qemu-devel] Question about vNVDIMM file format

From: Xiao Guangrong
Subject: Re: [Qemu-devel] Question about vNVDIMM file format
Date: Wed, 18 May 2016 15:04:52 +0800
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.6.0

Hi Rich,

On 05/17/2016 02:25 AM, Richard W.M. Jones wrote:
On Mon, May 16, 2016 at 09:53:36AM -0700, Stefan Hajnoczi wrote:
On Mon, May 16, 2016 at 04:04:01PM +0100, Richard W.M. Jones wrote:
I'm playing with ext4 and DAX.

I'm using:

   -object memory-backend-file,id=mem1,share,mem-path=/var/tmp/pmem,size=4G \
   -device nvdimm,memdev=mem1,id=nv1

where /var/tmp/pmem is a 4 GB ext4 filesystem image (no partition
table).  I can mount this in the guest using:

   mount -o dax /dev/pmem0 /mnt

and everything appears to work.

I read in the mailing list that the pmem file has some internal
structure for storing config data, stored in the last 128 KB of the
file.  Is that still the case?

AFAICT qemu.git/master does not support the ACPI _DSM for namespace
configuration.  That means the entire /var/tmp/pmem should be visible.

That's great, thanks both for your answers.

FWIW I was able to add support to libguestfs -- at least for the
"direct" backend where we run qemu directly.  Unfortunately libvirt
does not support the vNVDIMM device yet.

I have posted the two patches needed on our mailing list.  There seems
to be some delay in our mail server, so they aren't in the archives


There are a few possible problems / questions I have:

(a) How necessary is the ACPI dependency?  We disable ACPI because it
is quite slow, adding something like 150-200ms to the boot process
(every millisecond counts for us!).  Because I previously never needed
ACPI, I never really looked into why this is, and it could be
something quite simple, so I'm going to look at this issue next.  I
understand that NVDIMMs are not regular (eg) PCI devices, so ordinary
device probing isn't going to work, and that probably answers the
question why you need to use ACPI.

Yes, ACPI is necessary to export NVDIMM devices. The good news is that
Intel is working on ‘lite QEMU’ which only has basic/simplest ACPI
support. Haozhong, who has been CCed, is working on it.

(b) Could you describe what the 3 modules (nd_btt, nd_pmem, nfit) do?
Are all 3 modules necessary in the guest kernel?

I think the best answer is from Kernel's Kconfig :):
ACPI_NFIT: Infrastructure to probe ACPI 6 compliant platforms for
           NVDIMMs (NFIT) and register a libnvdimm device tree

          The Block Translation Table (BTT) provides atomic sector
          update semantics for persistent memory devices, so that
          applications that rely on sector writes not being torn (a
          guarantee that typical disks provide) can continue to do so.

          Memory ranges for PMEM are described by either an NFIT
          (NVDIMM Firmware Interface Table, see CONFIG_NFIT_ACPI), a
          non-standard OEM-specific E820 memory type (type-12, see
          CONFIG_X86_PMEM_LEGACY), or it is manually specified by the
          'memmap=nn[KMG]!ss[KMG]' kernel command line (see
          Documentation/kernel-parameters.txt).  This driver converts
          these persistent memory ranges into block devices that are
          capable of DAX (direct-access) file system mappings

Currently vNVDIMM is pure PMEM device without label, BTT is unnecessary,
so you can say N to BTT when configuring linux kernel for VM.

(c) I've got the root filesystem (which is actually ext2, but using
the ext4.ko driver) mounted with -o dax.  What benefits / differences
should I observe?  Just general reduced memory / page cache usage?

And better performance as slow IO path is not needed anymore. :)

However, there is potential issue if it is not backend by real NVDIMM
hardware, the data is not persistent. We are going to resolve it by
emulating PCOMMIT and do msync properly.

(d) If, in future, you add the namespace metadata, what tools will be
available on the host to create a packed filesystem + metadata?
Assuming that we won't be able to export just a filesystem as I am
doing now.

Yes, this kind of tool is useful, we has this plan however it is low priority
in our TODO. :(

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