exec: introduce MemoryRegionCache

Device models often have to perform multiple access to a single
memory region that is known in advance, but would to use "DMA-style"
functions instead of address_space_map/unmap.  This can happen
for example when the data has to undergo endianness conversion.
Introduce a new data structure to cache the result of
address_space_translate without forcing usage of a host address
like address_space_map does.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

exec.c

@@ -3077,6 +3077,82 @@ void cpu_physical_memory_unmap(void *buffer, hwaddr len,
 #define RCU_READ_UNLOCK(...)     rcu_read_unlock()
 #include "memory_ldst.inc.c"
 
+int64_t address_space_cache_init(MemoryRegionCache *cache,
+                                 AddressSpace *as,
+                                 hwaddr addr,
+                                 hwaddr len,
+                                 bool is_write)
+{
+    hwaddr l, xlat;
+    MemoryRegion *mr;
+    void *ptr;
+
+    assert(len > 0);
+
+    l = len;
+    mr = address_space_translate(as, addr, &xlat, &l, is_write);
+    if (!memory_access_is_direct(mr, is_write)) {
+        return -EINVAL;
+    }
+
+    l = address_space_extend_translation(as, addr, len, mr, xlat, l, is_write);
+    ptr = qemu_ram_ptr_length(mr->ram_block, xlat, &l);
+
+    cache->xlat = xlat;
+    cache->is_write = is_write;
+    cache->mr = mr;
+    cache->ptr = ptr;
+    cache->len = l;
+    memory_region_ref(cache->mr);
+
+    return l;
+}
+
+void address_space_cache_invalidate(MemoryRegionCache *cache,
+                                    hwaddr addr,
+                                    hwaddr access_len)
+{
+    assert(cache->is_write);
+    invalidate_and_set_dirty(cache->mr, addr + cache->xlat, access_len);
+}
+
+void address_space_cache_destroy(MemoryRegionCache *cache)
+{
+    if (!cache->mr) {
+        return;
+    }
+
+    if (xen_enabled()) {
+        xen_invalidate_map_cache_entry(cache->ptr);
+    }
+    memory_region_unref(cache->mr);
+}
+
+/* Called from RCU critical section.  This function has the same
+ * semantics as address_space_translate, but it only works on a
+ * predefined range of a MemoryRegion that was mapped with
+ * address_space_cache_init.
+ */
+static inline MemoryRegion *address_space_translate_cached(
+    MemoryRegionCache *cache, hwaddr addr, hwaddr *xlat,
+    hwaddr *plen, bool is_write)
+{
+    assert(addr < cache->len && *plen <= cache->len - addr);
+    *xlat = addr + cache->xlat;
+    return cache->mr;
+}
+
+#define ARG1_DECL                MemoryRegionCache *cache
+#define ARG1                     cache
+#define SUFFIX                   _cached
+#define TRANSLATE(...)           address_space_translate_cached(cache, __VA_ARGS__)
+#define IS_DIRECT(mr, is_write)  true
+#define MAP_RAM(mr, ofs)         (cache->ptr + (ofs - cache->xlat))
+#define INVALIDATE(mr, ofs, len) ((void)0)
+#define RCU_READ_LOCK()          ((void)0)
+#define RCU_READ_UNLOCK()        ((void)0)
+#include "memory_ldst.inc.c"
+
 /* virtual memory access for debug (includes writing to ROM) */
 int cpu_memory_rw_debug(CPUState *cpu, target_ulong addr,
                         uint8_t *buf, int len, int is_write)