spapr: Add rtas_st_buffer utility function

Add a function to write lengh + data into a buffer as required for the
emulation of the RTAS ibm,get-system-parameter call.

If the destination is smaller than the source, the write is truncated
and success is returned. This matches the behaviour of pHyp.

This will be used in following patches.
Signed-off-by: NSam Bobroff <sam.bobroff@au1.ibm.com>
Signed-off-by: NAlexander Graf <agraf@suse.de>

include/hw/ppc/spapr.h

@@ -412,6 +412,19 @@ static inline void rtas_st(target_ulong phys, int n, uint32_t val)
     stl_be_phys(&address_space_memory, ppc64_phys_to_real(phys + 4*n), val);
 }
 
+
+static inline void rtas_st_buffer(target_ulong phys, target_ulong phys_len,
+                                  uint8_t *buffer, uint16_t buffer_len)
+{
+    if (phys_len < 2) {
+        return;
+    }
+    stw_be_phys(&address_space_memory,
+                ppc64_phys_to_real(phys), buffer_len);
+    cpu_physical_memory_write(ppc64_phys_to_real(phys + 2),
+                              buffer, MIN(buffer_len, phys_len - 2));
+}
+
 typedef void (*spapr_rtas_fn)(PowerPCCPU *cpu, sPAPREnvironment *spapr,
                               uint32_t token,
                               uint32_t nargs, target_ulong args,