[PATCH v2 3/3] nvdimm: honor -object memory-backend-file, readonly=on option

[Stefan Hajnoczi <stefanha@redhat.com>]

Make it possible to present read-only files to the guest as "unarmed"
NVDIMMs. The Linux NVDIMM device (/dev/pmemX) is read-only.

Acked-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
---
 docs/nvdimm.txt | 8 +++++++-
 hw/mem/nvdimm.c | 4 ++++
 2 files changed, 11 insertions(+), 1 deletion(-)

diff --git a/docs/nvdimm.txt b/docs/nvdimm.txt
index c2c6e441b3..06c2008107 100644
--- a/docs/nvdimm.txt
+++ b/docs/nvdimm.txt
@@ -17,7 +17,7 @@ following command line options:
 
  -machine pc,nvdimm
  -m $RAM_SIZE,slots=$N,maxmem=$MAX_SIZE
- -object memory-backend-file,id=mem1,share=on,mem-path=$PATH,size=$NVDIMM_SIZE
+ -object memory-backend-file,id=mem1,share=on,mem-path=$PATH,size=$NVDIMM_SIZE,readonly=off
  -device nvdimm,id=nvdimm1,memdev=mem1
 
 Where,
@@ -42,6 +42,12 @@ Where,
    "share=off", then guest writes won't be applied to the backend
    file and thus will be invisible to other guests.
 
+   "readonly=on/off" controls whether the file $PATH is opened read-only or
+   read/write (default). "readonly=on" sets the ACPI NFIT NVDIMM Region Mapping
+   Structure "NVDIMM State Flags" Bit 3 indicating that the device is "unarmed"
+   and cannot accept persistent writes. Linux guest drivers set the device to
+   read-only when this bit is present.
+
  - "device nvdimm,id=nvdimm1,memdev=mem1" creates a virtual NVDIMM
    device whose storage is provided by above memory backend device.
 
diff --git a/hw/mem/nvdimm.c b/hw/mem/nvdimm.c
index e1574bc07c..848cd65917 100644
--- a/hw/mem/nvdimm.c
+++ b/hw/mem/nvdimm.c
@@ -146,6 +146,10 @@ static void nvdimm_prepare_memory_region(NVDIMMDevice *nvdimm, Error **errp)
         return;
     }
 
+    if (memory_region_is_rom(mr)) {
+        nvdimm->unarmed = true; /* this device is read-only */
+    }
+
     nvdimm->nvdimm_mr = g_new(MemoryRegion, 1);
     memory_region_init_alias(nvdimm->nvdimm_mr, OBJECT(dimm),
                              "nvdimm-memory", mr, 0, pmem_size);
-- 
2.26.2