Add support for H_SCM_HEALTH hcall described at [1] for spapr
nvdimms. This enables guest to detect the 'unarmed' status of a
specific spapr nvdimm identified by its DRC and if its unarmed, mark
the region backed by the nvdimm as read-only.

The patch adds h_scm_health() to handle the H_SCM_HEALTH hcall which
returns two 64-bit bitmaps (health bitmap, health bitmap mask) derived
from 'struct nvdimm->unarmed' member.

Linux kernel side changes to enable handling of 'unarmed' nvdimms for
ppc64 are proposed at [2].

References:
[1] "Hypercall Op-codes (hcalls)"
    https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/Documentation/powerpc/papr_hcalls.rst#n220
[2] "powerpc/papr_scm: Mark nvdimm as unarmed if needed during probe"
    https://lore.kernel.org/linux-nvdimm/20210329113103.476760-1-vaibhav@linux.ibm.com/

Signed-off-by: Vaibhav Jain <vaibhav@linux.ibm.com>
---
Changelog

v2:
* Added a check for drc->dev to ensure that the dimm is plugged in
  when servicing H_SCM_HEALTH. [ Shiva ]
* Instead of accessing the 'nvdimm->unarmed' member directly use the
  object_property_get_bool accessor to fetch it. [ Shiva ]
* Update the usage of PAPR_PMEM_UNARMED* macros [ Greg ]
* Updated patch description reference#1 to point appropriate section
  in the documentation. [ Greg ]
---
 hw/ppc/spapr_nvdimm.c  | 38 ++++++++++++++++++++++++++++++++++++++
 include/hw/ppc/spapr.h |  3 ++-
 2 files changed, 40 insertions(+), 1 deletion(-)

diff --git a/hw/ppc/spapr_nvdimm.c b/hw/ppc/spapr_nvdimm.c
index b46c36917c..34096e4718 100644
--- a/hw/ppc/spapr_nvdimm.c
+++ b/hw/ppc/spapr_nvdimm.c
@@ -31,6 +31,13 @@
 #include "qemu/range.h"
 #include "hw/ppc/spapr_numa.h"
 
+/* DIMM health bitmap bitmap indicators. Taken from kernel's papr_scm.c */
+/* SCM device is unable to persist memory contents */
+#define PAPR_PMEM_UNARMED (1ULL << (63 - 0))
+
+/* Bits status indicators for health bitmap indicating unarmed dimm */
+#define PAPR_PMEM_UNARMED_MASK (PAPR_PMEM_UNARMED)
+
 bool spapr_nvdimm_validate(HotplugHandler *hotplug_dev, NVDIMMDevice *nvdimm,
                            uint64_t size, Error **errp)
 {
@@ -467,6 +474,36 @@ static target_ulong h_scm_unbind_all(PowerPCCPU *cpu, SpaprMachineState *spapr,
     return H_SUCCESS;
 }
 
+static target_ulong h_scm_health(PowerPCCPU *cpu, SpaprMachineState *spapr,
+                                 target_ulong opcode, target_ulong *args)
+{
+    uint32_t drc_index = args[0];
+    SpaprDrc *drc = spapr_drc_by_index(drc_index);
+    NVDIMMDevice *nvdimm;
+
+    if (drc && spapr_drc_type(drc) != SPAPR_DR_CONNECTOR_TYPE_PMEM) {
+        return H_PARAMETER;
+    }
+
+    /* Ensure that the dimm is plugged in */
+    if (!drc->dev) {
+        return H_HARDWARE;
+    }
+
+    nvdimm = NVDIMM(drc->dev);
+
+    args[0] = 0;
+    /* Check if the nvdimm is unarmed and send its status via health bitmaps */
+    if (object_property_get_bool(OBJECT(nvdimm), NVDIMM_UNARMED_PROP, NULL)) {
+        args[0] |= PAPR_PMEM_UNARMED;
+    }
+
+    /* Update the health bitmap with the applicable mask */
+    args[1] = PAPR_PMEM_UNARMED_MASK;
+
+    return H_SUCCESS;
+}
+
 static void spapr_scm_register_types(void)
 {
     /* qemu/scm specific hcalls */
@@ -475,6 +512,7 @@ static void spapr_scm_register_types(void)
     spapr_register_hypercall(H_SCM_BIND_MEM, h_scm_bind_mem);
     spapr_register_hypercall(H_SCM_UNBIND_MEM, h_scm_unbind_mem);
     spapr_register_hypercall(H_SCM_UNBIND_ALL, h_scm_unbind_all);
+    spapr_register_hypercall(H_SCM_HEALTH, h_scm_health);
 }
 
 type_init(spapr_scm_register_types)
diff --git a/include/hw/ppc/spapr.h b/include/hw/ppc/spapr.h
index 47cebaf3ac..6e1eafb05d 100644
--- a/include/hw/ppc/spapr.h
+++ b/include/hw/ppc/spapr.h
@@ -538,8 +538,9 @@ struct SpaprMachineState {
 #define H_SCM_BIND_MEM          0x3EC
 #define H_SCM_UNBIND_MEM        0x3F0
 #define H_SCM_UNBIND_ALL        0x3FC
+#define H_SCM_HEALTH            0x400
 
-#define MAX_HCALL_OPCODE        H_SCM_UNBIND_ALL
+#define MAX_HCALL_OPCODE        H_SCM_HEALTH
 
 /* The hcalls above are standardized in PAPR and implemented by pHyp
  * as well.
-- 
2.30.2