target/riscv: Reorg sum check in get_physical_address

Implement this by adjusting prot, which reduces the set of
checks required.  This prevents exec to be set for U pages
in MMUIdx_S_SUM.  While it had been technically incorrect,
it did not manifest as a bug, because we will never attempt
to execute from MMUIdx_S_SUM.

Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Weiwei Li <liweiwei@iscas.ac.cn>
Tested-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Message-Id: <20230325105429.1142530-26-richard.henderson@linaro.org>
Message-Id: <20230412114333.118895-26-richard.henderson@linaro.org>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>

target/riscv/cpu_helper.c

@@ -786,7 +786,7 @@ static int get_physical_address(CPURISCVState *env, hwaddr *physical,
     *ret_prot = 0;
 
     hwaddr base;
-    int levels, ptidxbits, ptesize, vm, sum, widened;
+    int levels, ptidxbits, ptesize, vm, widened;
 
     if (first_stage == true) {
         if (use_background) {
@@ -817,7 +817,7 @@ static int get_physical_address(CPURISCVState *env, hwaddr *physical,
         }
         widened = 2;
     }
-    sum = mmuidx_sum(mmu_idx);
+
     switch (vm) {
     case VM_1_10_SV32:
       levels = 2; ptidxbits = 10; ptesize = 4; break;
@@ -985,15 +985,15 @@ static int get_physical_address(CPURISCVState *env, hwaddr *physical,
         prot |= PAGE_EXEC;
     }
 
-    if ((pte & PTE_U) &&
-        ((mode != PRV_U) && (!sum || access_type == MMU_INST_FETCH))) {
-        /*
-         * User PTE flags when not U mode and mstatus.SUM is not set,
-         * or the access type is an instruction fetch.
-         */
-        return TRANSLATE_FAIL;
-    }
-    if (!(pte & PTE_U) && (mode != PRV_S)) {
+    if (pte & PTE_U) {
+        if (mode != PRV_U) {
+            if (!mmuidx_sum(mmu_idx)) {
+                return TRANSLATE_FAIL;
+            }
+            /* SUM allows only read+write, not execute. */
+            prot &= PAGE_READ | PAGE_WRITE;
+        }
+    } else if (mode != PRV_S) {
         /* Supervisor PTE flags when not S mode */
         return TRANSLATE_FAIL;
     }