Earlier since the SBE_CMD_STASH_MPIPL_CONFIG command was not handled, so
skiboot used to not get any response from SBE:

    [  106.350742821,3] SBE: Message timeout [chip id = 0], cmd = d7, subcmd = 7
    [  106.352067746,3] SBE: Failed to send stash MPIPL config [chip id = 0x0, rc = 254]

Fix this by handling the command in PowerNV SBE, and sending a response so
skiboot knows SBE has handled the STASH command

The stashed skiboot base is later used to access the relocated MDST/MDDT
tables when MPIPL is implemented.

The purpose of stashing relocated base address is explained in following
skiboot commit:

    author Vasant Hegde <hegdevasant@linux.vnet.ibm.com> Fri Jul 12 16:47:51 2019 +0530
    committer Oliver O'Halloran <oohall@gmail.com> Thu Aug 15 17:53:39 2019 +1000

    SBE: Send OPAL relocated base address to SBE

      OPAL relocates itself during boot. During memory preserving IPL hostboot needs
      to access relocated OPAL base address to get MDST, MDDT tables. Hence send
      relocated base address to SBE via 'stash MPIPL config' chip-op. During next
      IPL SBE will send stashed data to hostboot... so that hostboot can access
      these data.

Signed-off-by: Aditya Gupta <adityag@linux.ibm.com>
---
 hw/ppc/pnv_sbe.c           | 27 +++++++++++++++++++++++++++
 include/hw/ppc/pnv_mpipl.h |  3 +++
 2 files changed, 30 insertions(+)

diff --git a/hw/ppc/pnv_sbe.c b/hw/ppc/pnv_sbe.c
index d004b7d5c225..af888126e758 100644
--- a/hw/ppc/pnv_sbe.c
+++ b/hw/ppc/pnv_sbe.c
@@ -233,6 +233,7 @@ static void sbe_timer(void *opaque)
 
 static void do_sbe_msg(PnvSBE *sbe)
 {
+    PnvMachineState *pnv = PNV_MACHINE(qdev_get_machine());
     struct sbe_msg msg;
     uint16_t cmd, ctrl_flags, seq_id;
     int i;
@@ -265,6 +266,32 @@ static void do_sbe_msg(PnvSBE *sbe)
             timer_del(sbe->timer);
         }
         break;
+    case SBE_CMD_STASH_MPIPL_CONFIG:
+        /* key = sbe->mbox[1] */
+        switch (sbe->mbox[1]) {
+        case SBE_STASH_KEY_SKIBOOT_BASE:
+            pnv->mpipl_state.skiboot_base = sbe->mbox[2];
+            qemu_log_mask(LOG_UNIMP,
+                "Stashing skiboot base: 0x%" HWADDR_PRIx "\n",
+                pnv->mpipl_state.skiboot_base);
+
+            /*
+             * Set the response register.
+             *
+             * Currently setting the same sequence number in
+             * response as we got in the request.
+             */
+            sbe->mbox[4] = sbe->mbox[0];    /* sequence number */
+            pnv_sbe_set_host_doorbell(sbe,
+                    sbe->host_doorbell | SBE_HOST_RESPONSE_WAITING);
+
+            break;
+        default:
+            qemu_log_mask(LOG_UNIMP,
+                "SBE: CMD_STASH_MPIPL_CONFIG: Unimplemented key: 0x" TARGET_FMT_lx "\n",
+                sbe->mbox[1]);
+        }
+        break;
     default:
         qemu_log_mask(LOG_UNIMP, "SBE Unimplemented command: 0x%x\n", cmd);
     }
diff --git a/include/hw/ppc/pnv_mpipl.h b/include/hw/ppc/pnv_mpipl.h
index c544984dc76d..60d6ede48209 100644
--- a/include/hw/ppc/pnv_mpipl.h
+++ b/include/hw/ppc/pnv_mpipl.h
@@ -8,11 +8,14 @@
 #define PNV_MPIPL_H
 
 #include "qemu/osdep.h"
+#include "exec/hwaddr.h"
 
 typedef struct MpiplPreservedState MpiplPreservedState;
 
 /* Preserved state to be saved in PnvMachineState */
 struct MpiplPreservedState {
+    /* skiboot_base will be valid only after OPAL sends relocated base to SBE */
+    hwaddr     skiboot_base;
     bool       is_next_boot_mpipl;
 };
 
-- 
2.52.0