RISC-V: Improve page table walker spec compliance

- Inline PTE_TABLE check for better readability
- Change access checks from ternary operator to if
- Improve readibility of User page U mode and SUM test
- Disallow non U mode from fetching from User pages
- Add reserved PTE flag check: W or W|X
- Add misaligned PPN check
- Set READ protection for PTE X flag and mstatus.mxr
- Use memory_region_is_ram in pte update

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: Michael Clark <mjc@sifive.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>

target/riscv/cpu_bits.h

@@ -407,5 +407,3 @@
 #define PTE_SOFT  0x300 /* Reserved for Software */
 
 #define PTE_PPN_SHIFT 10
-
-#define PTE_TABLE(PTE) (((PTE) & (PTE_V | PTE_R | PTE_W | PTE_X)) == PTE_V)

target/riscv/helper.c

@@ -185,16 +185,39 @@ restart:
 #endif
         target_ulong ppn = pte >> PTE_PPN_SHIFT;
 
-        if (PTE_TABLE(pte)) { /* next level of page table */
+        if (!(pte & PTE_V)) {
+            /* Invalid PTE */
+            return TRANSLATE_FAIL;
+        } else if (!(pte & (PTE_R | PTE_W | PTE_X))) {
+            /* Inner PTE, continue walking */
             base = ppn << PGSHIFT;
-        } else if ((pte & PTE_U) ? (mode == PRV_S) && !sum : !(mode == PRV_S)) {
+        } else if ((pte & (PTE_R | PTE_W | PTE_X)) == PTE_W) {
-            break;
+            /* Reserved leaf PTE flags: PTE_W */
-        } else if (!(pte & PTE_V) || (!(pte & PTE_R) && (pte & PTE_W))) {
+            return TRANSLATE_FAIL;
-            break;
+        } else if ((pte & (PTE_R | PTE_W | PTE_X)) == (PTE_W | PTE_X)) {
-        } else if (access_type == MMU_INST_FETCH ? !(pte & PTE_X) :
+            /* Reserved leaf PTE flags: PTE_W + PTE_X */
-                  access_type == MMU_DATA_LOAD ?  !(pte & PTE_R) &&
+            return TRANSLATE_FAIL;
-                  !(mxr && (pte & PTE_X)) : !((pte & PTE_R) && (pte & PTE_W))) {
+        } else if ((pte & PTE_U) && ((mode != PRV_U) &&
-            break;
+                   (!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;
+        } else if (!(pte & PTE_U) && (mode != PRV_S)) {
+            /* Supervisor PTE flags when not S mode */
+            return TRANSLATE_FAIL;
+        } else if (ppn & ((1ULL << ptshift) - 1)) {
+            /* Misaligned PPN */
+            return TRANSLATE_FAIL;
+        } else if (access_type == MMU_DATA_LOAD && !((pte & PTE_R) ||
+                   ((pte & PTE_X) && mxr))) {
+            /* Read access check failed */
+            return TRANSLATE_FAIL;
+        } else if (access_type == MMU_DATA_STORE && !(pte & PTE_W)) {
+            /* Write access check failed */
+            return TRANSLATE_FAIL;
+        } else if (access_type == MMU_INST_FETCH && !(pte & PTE_X)) {
+            /* Fetch access check failed */
+            return TRANSLATE_FAIL;
         } else {
             /* if necessary, set accessed and dirty bits. */
             target_ulong updated_pte = pte | PTE_A |
@@ -202,16 +225,19 @@ restart:
 
             /* Page table updates need to be atomic with MTTCG enabled */
             if (updated_pte != pte) {
-                /* if accessed or dirty bits need updating, and the PTE is
+                /*
-                 * in RAM, then we do so atomically with a compare and swap.
+                 * - if accessed or dirty bits need updating, and the PTE is
-                 * if the PTE is in IO space, then it can't be updated.
+                 *   in RAM, then we do so atomically with a compare and swap.
-                 * if the PTE changed, then we must re-walk the page table
+                 * - if the PTE is in IO space or ROM, then it can't be updated
-                   as the PTE is no longer valid */
+                 *   and we return TRANSLATE_FAIL.
+                 * - if the PTE changed by the time we went to update it, then
+                 *   it is no longer valid and we must re-walk the page table.
+                 */
                 MemoryRegion *mr;
                 hwaddr l = sizeof(target_ulong), addr1;
                 mr = address_space_translate(cs->as, pte_addr,
                     &addr1, &l, false, MEMTXATTRS_UNSPECIFIED);
-                if (memory_access_is_direct(mr, true)) {
+                if (memory_region_is_ram(mr)) {
                     target_ulong *pte_pa =
                         qemu_map_ram_ptr(mr->ram_block, addr1);
 #if TCG_OVERSIZED_GUEST
@@ -239,15 +265,15 @@ restart:
             target_ulong vpn = addr >> PGSHIFT;
             *physical = (ppn | (vpn & ((1L << ptshift) - 1))) << PGSHIFT;
 
-            if ((pte & PTE_R)) {
+            /* set permissions on the TLB entry */
+            if ((pte & PTE_R) || ((pte & PTE_X) && mxr)) {
                 *prot |= PAGE_READ;
             }
             if ((pte & PTE_X)) {
                 *prot |= PAGE_EXEC;
             }
-           /* only add write permission on stores or if the page
+            /* add write permission on stores or if the page is already dirty,
-              is already dirty, so that we don't miss further
+               so that we TLB miss on later writes to update the dirty bit */
-              page table walks to update the dirty bit */
             if ((pte & PTE_W) &&
                     (access_type == MMU_DATA_STORE || (pte & PTE_D))) {
                 *prot |= PAGE_WRITE;