RISC-V: Allow S-mode mxr access when priv ISA >= v1.10

The mstatus.MXR alias in sstatus should only be writable
by S-mode if the privileged ISA version >= v1.10. Also MXR
was masked in sstatus CSR read but not sstatus CSR writes.
Now we correctly mask sstatus.mxr in both read and write.

Cc: Sagar Karandikar <sagark@eecs.berkeley.edu>
Cc: Bastian Koppelmann <kbastian@mail.uni-paderborn.de>
Cc: Palmer Dabbelt <palmer@sifive.com>
Cc: Alistair Francis <Alistair.Francis@wdc.com>
Signed-off-by: NMichael Clark <mjc@sifive.com>
Reviewed-by: NAlistair Francis <alistair.francis@wdc.com>

target/riscv/op_helper.c

@@ -234,7 +234,10 @@ void csr_write_helper(CPURISCVState *env, target_ulong val_to_write,
         target_ulong ms = env->mstatus;
         target_ulong mask = SSTATUS_SIE | SSTATUS_SPIE | SSTATUS_UIE
             | SSTATUS_UPIE | SSTATUS_SPP | SSTATUS_FS | SSTATUS_XS
-            | SSTATUS_SUM | SSTATUS_MXR | SSTATUS_SD;
+            | SSTATUS_SUM | SSTATUS_SD;
+        if (env->priv_ver >= PRIV_VERSION_1_10_0) {
+            mask |= SSTATUS_MXR;
+        }
         ms = (ms & ~mask) | (val_to_write & mask);
         csr_write_helper(env, ms, CSR_MSTATUS);
         break;
@@ -441,7 +444,7 @@ target_ulong csr_read_helper(CPURISCVState *env, target_ulong csrno)
     case CSR_SSTATUS: {
         target_ulong mask = SSTATUS_SIE | SSTATUS_SPIE | SSTATUS_UIE
             | SSTATUS_UPIE | SSTATUS_SPP | SSTATUS_FS | SSTATUS_XS
-            | SSTATUS_SUM |  SSTATUS_SD;
+            | SSTATUS_SUM | SSTATUS_SD;
         if (env->priv_ver >= PRIV_VERSION_1_10_0) {
             mask |= SSTATUS_MXR;
         }