target/loongarch: Fix page size set issue with CSR_STLBPS

When modify register CSR_STLBPS, the page size should come from
input parameter rather than old value.

Signed-off-by: Bibo Mao <maobibo@loongson.cn>
Reviewed-by: Song Gao <gaosong@loongson.cn>

Author: bibo-mao

target/loongarch/cpu-csr.h

@@ -106,6 +106,7 @@ FIELD(CSR_PWCH, DIR4_WIDTH, 18, 6)
 
 #define LOONGARCH_CSR_STLBPS         0x1e /* Stlb page size */
 FIELD(CSR_STLBPS, PS, 0, 5)
+FIELD(CSR_STLBPS, RESERVE, 5, 27)
 
 #define LOONGARCH_CSR_RVACFG         0x1f /* Reduced virtual address config */
 FIELD(CSR_RVACFG, RBITS, 0, 4)

target/loongarch/tcg/csr_helper.c

@@ -26,13 +26,14 @@ target_ulong helper_csrwr_stlbps(CPULoongArchState *env, target_ulong val)
      * The real hardware only supports the min tlb_ps is 12
      * tlb_ps=0 may cause undefined-behavior.
      */
-    uint8_t tlb_ps = FIELD_EX64(env->CSR_STLBPS, CSR_STLBPS, PS);
+    uint8_t tlb_ps = FIELD_EX64(val, CSR_STLBPS, PS);
     if (!check_ps(env, tlb_ps)) {
         qemu_log_mask(LOG_GUEST_ERROR,
                       "Attempted set ps %d\n", tlb_ps);
     } else {
         /* Only update PS field, reserved bit keeps zero */
-        env->CSR_STLBPS = FIELD_DP64(old_v, CSR_STLBPS, PS, tlb_ps);
+        val = FIELD_DP64(val, CSR_STLBPS, RESERVE, 0);
+        env->CSR_STLBPS = val;
     }
 
     return old_v;