Merge remote-tracking branch 'remotes/ehabkost/tags/machine-next-pull-request' into staging

machine queue, 2018-01-19

# gpg: Signature made Fri 19 Jan 2018 16:30:19 GMT
# gpg:                using RSA key 0x2807936F984DC5A6
# gpg: Good signature from "Eduardo Habkost <ehabkost@redhat.com>"
# Primary key fingerprint: 5A32 2FD5 ABC4 D3DB ACCF  D1AA 2807 936F 984D C5A6

* remotes/ehabkost/tags/machine-next-pull-request:
  fw_cfg: fix memory corruption when all fw_cfg slots are used
  possible_cpus: add CPUArchId::type field
  nvdimm: add 'unarmed' option
  nvdimm: add a macro for property "label-size"
  hostmem-file: add "align" option
  scripts: Remove fixed entries from the device-crash-test
  qdev: Check for the availability of a hotplug controller before adding a device
  qdev_monitor: Simplify error handling in qdev_device_add()
  q35: Allow only supported dynamic sysbus devices
  xen: Add only xen-sysdev to dynamic sysbus device list
  spapr: Allow only supported dynamic sysbus devices
  ppc: e500: Allow only supported dynamic sysbus devices
  hw/arm/virt: Allow only supported dynamic sysbus devices
  machine: Replace has_dynamic_sysbus with list of allowed devices
  numa: fix missing '-numa cpu' in '-help' output
  qemu-options: document memory-backend-ram
  qemu-options: document missing memory-backend-file options
  memfd: remove needless include
  memfd: split qemu_memfd_alloc()

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

1 files changed, 31 insertions, 0 deletions

diff --git a/docs/nvdimm.txt b/docs/nvdimm.txt
index 2d9f8c0e8c..e903d8bb09 100644
--- a/docs/nvdimm.txt
+++ b/docs/nvdimm.txt
@@ -122,3 +122,34 @@ Note:
      M >= size of RAM devices +
           size of statically plugged vNVDIMM devices +
           size of hotplugged vNVDIMM devices
+
+Alignment
+---------
+
+QEMU uses mmap(2) to maps vNVDIMM backends and aligns the mapping
+address to the page size (getpagesize(2)) by default. However, some
+types of backends may require an alignment different than the page
+size. In that case, QEMU v2.12.0 and later provide 'align' option to
+memory-backend-file to allow users to specify the proper alignment.
+
+For example, device dax require the 2 MB alignment, so we can use
+following QEMU command line options to use it (/dev/dax0.0) as the
+backend of vNVDIMM:
+
+ -object memory-backend-file,id=mem1,share=on,mem-path=/dev/dax0.0,size=4G,align=2M
+ -device nvdimm,id=nvdimm1,memdev=mem1
+
+Guest Data Persistence
+----------------------
+
+Though QEMU supports multiple types of vNVDIMM backends on Linux,
+currently the only one that can guarantee the guest write persistence
+is the device DAX on the real NVDIMM device (e.g., /dev/dax0.0), to
+which all guest access do not involve any host-side kernel cache.
+
+When using other types of backends, it's suggested to set 'unarmed'
+option of '-device nvdimm' to 'on', which sets the unarmed flag of the
+guest NVDIMM region mapping structure.  This unarmed flag indicates
+guest software that this vNVDIMM device contains a region that cannot
+accept persistent writes. In result, for example, the guest Linux
+NVDIMM driver, marks such vNVDIMM device as read-only.