target/arm: New function allocation_tag_mem_probe()

For the FEAT_MOPS operations, the existing allocation_tag_mem()
function almost does what we want, but it will take a watchpoint
exception even for an ra == 0 probe request, and it requires that the
caller guarantee that the memory is accessible.  For FEAT_MOPS we
want a function that will not take any kind of exception, and will
return NULL for the not-accessible case.

Rename allocation_tag_mem() to allocation_tag_mem_probe() and add an
extra 'probe' argument that lets us distinguish these cases;
allocation_tag_mem() is now a wrapper that always passes 'false'.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20230912140434.1333369-6-peter.maydell@linaro.org

target/arm/tcg/mte_helper.c

@@ -50,13 +50,14 @@ static int choose_nonexcluded_tag(int tag, int offset, uint16_t exclude)
 }
 
 /**
- * allocation_tag_mem:
+ * allocation_tag_mem_probe:
  * @env: the cpu environment
  * @ptr_mmu_idx: the addressing regime to use for the virtual address
  * @ptr: the virtual address for which to look up tag memory
  * @ptr_access: the access to use for the virtual address
  * @ptr_size: the number of bytes in the normal memory access
  * @tag_access: the access to use for the tag memory
+ * @probe: true to merely probe, never taking an exception
  * @ra: the return address for exception handling
  *
  * Our tag memory is formatted as a sequence of little-endian nibbles.
@@ -65,22 +66,34 @@ static int choose_nonexcluded_tag(int tag, int offset, uint16_t exclude)
  * for the higher addr.
  *
  * Here, resolve the physical address from the virtual address, and return
- * a pointer to the corresponding tag byte.  Exit with exception if the
- * virtual address is not accessible for @ptr_access.
+ * a pointer to the corresponding tag byte.
  *
  * If there is no tag storage corresponding to @ptr, return NULL.
+ *
+ * If the page is inaccessible for @ptr_access, or has a watchpoint, there are
+ * three options:
+ * (1) probe = true, ra = 0 : pure probe -- we return NULL if the page is not
+ *     accessible, and do not take watchpoint traps. The calling code must
+ *     handle those cases in the right priority compared to MTE traps.
+ * (2) probe = false, ra = 0 : probe, no fault expected -- the caller guarantees
+ *     that the page is going to be accessible. We will take watchpoint traps.
+ * (3) probe = false, ra != 0 : non-probe -- we will take both memory access
+ *     traps and watchpoint traps.
+ * (probe = true, ra != 0 is invalid and will assert.)
  */
-static uint8_t *allocation_tag_mem(CPUARMState *env, int ptr_mmu_idx,
-                                   uint64_t ptr, MMUAccessType ptr_access,
-                                   int ptr_size, MMUAccessType tag_access,
-                                   uintptr_t ra)
+static uint8_t *allocation_tag_mem_probe(CPUARMState *env, int ptr_mmu_idx,
+                                         uint64_t ptr, MMUAccessType ptr_access,
+                                         int ptr_size, MMUAccessType tag_access,
+                                         bool probe, uintptr_t ra)
 {
 #ifdef CONFIG_USER_ONLY
     uint64_t clean_ptr = useronly_clean_ptr(ptr);
     int flags = page_get_flags(clean_ptr);
     uint8_t *tags;
     uintptr_t index;
 
+    assert(!(probe && ra));
+
     if (!(flags & (ptr_access == MMU_DATA_STORE ? PAGE_WRITE_ORG : PAGE_READ))) {
         cpu_loop_exit_sigsegv(env_cpu(env), ptr, ptr_access,
                               !(flags & PAGE_VALID), ra);
@@ -111,12 +124,16 @@ static uint8_t *allocation_tag_mem(CPUARMState *env, int ptr_mmu_idx,
      * exception for inaccessible pages, and resolves the virtual address
      * into the softmmu tlb.
      *
-     * When RA == 0, this is for mte_probe.  The page is expected to be
-     * valid.  Indicate to probe_access_flags no-fault, then assert that
-     * we received a valid page.
+     * When RA == 0, this is either a pure probe or a no-fault-expected probe.
+     * Indicate to probe_access_flags no-fault, then either return NULL
+     * for the pure probe, or assert that we received a valid page for the
+     * no-fault-expected probe.
      */
     flags = probe_access_full(env, ptr, 0, ptr_access, ptr_mmu_idx,
                               ra == 0, &host, &full, ra);
+    if (probe && (flags & TLB_INVALID_MASK)) {
+        return NULL;
+    }
     assert(!(flags & TLB_INVALID_MASK));
 
     /* If the virtual page MemAttr != Tagged, access unchecked. */
@@ -157,7 +174,7 @@ static uint8_t *allocation_tag_mem(CPUARMState *env, int ptr_mmu_idx,
     }
 
     /* Any debug exception has priority over a tag check exception. */
-    if (unlikely(flags & TLB_WATCHPOINT)) {
+    if (!probe && unlikely(flags & TLB_WATCHPOINT)) {
         int wp = ptr_access == MMU_DATA_LOAD ? BP_MEM_READ : BP_MEM_WRITE;
         assert(ra != 0);
         cpu_check_watchpoint(env_cpu(env), ptr, ptr_size, attrs, wp, ra);
@@ -199,6 +216,15 @@ static uint8_t *allocation_tag_mem(CPUARMState *env, int ptr_mmu_idx,
 #endif
 }
 
+static uint8_t *allocation_tag_mem(CPUARMState *env, int ptr_mmu_idx,
+                                   uint64_t ptr, MMUAccessType ptr_access,
+                                   int ptr_size, MMUAccessType tag_access,
+                                   uintptr_t ra)
+{
+    return allocation_tag_mem_probe(env, ptr_mmu_idx, ptr, ptr_access,
+                                    ptr_size, tag_access, false, ra);
+}
+
 uint64_t HELPER(irg)(CPUARMState *env, uint64_t rn, uint64_t rm)
 {
     uint16_t exclude = extract32(rm | env->cp15.gcr_el1, 0, 16);