RISC-V: Handle bus errors in the page table walker

We directly access physical memory while walking the page tables on
RISC-V, but while doing so we were using cpu_ld*() which does not report
bus errors.  This patch converts the page table walker over to use
address_space_ld*(), which allows bus errors to be detected.

Signed-off-by: Palmer Dabbelt <palmer@sifive.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: Palmer Dabbelt <palmer@sifive.com>

target/riscv/cpu_helper.c

@@ -169,7 +169,8 @@ static int get_physical_address(CPURISCVState *env, hwaddr *physical,
     /* NOTE: the env->pc value visible here will not be
      * correct, but the value visible to the exception handler
      * (riscv_cpu_do_interrupt) is correct */
-
+    MemTxResult res;
+    MemTxAttrs attrs = MEMTXATTRS_UNSPECIFIED;
     int mode = mmu_idx;
 
     if (mode == PRV_M && access_type != MMU_INST_FETCH) {
@@ -256,11 +257,16 @@ static int get_physical_address(CPURISCVState *env, hwaddr *physical,
             1 << MMU_DATA_LOAD, PRV_S)) {
             return TRANSLATE_PMP_FAIL;
         }
+
 #if defined(TARGET_RISCV32)
-        target_ulong pte = ldl_phys(cs->as, pte_addr);
+        target_ulong pte = address_space_ldl(cs->as, pte_addr, attrs, &res);
 #elif defined(TARGET_RISCV64)
-        target_ulong pte = ldq_phys(cs->as, pte_addr);
+        target_ulong pte = address_space_ldq(cs->as, pte_addr, attrs, &res);
 #endif
+        if (res != MEMTX_OK) {
+            return TRANSLATE_FAIL;
+        }
+
         hwaddr ppn = pte >> PTE_PPN_SHIFT;
 
         if (!(pte & PTE_V)) {