[PATCH v4] memory: Directly dispatch alias accesses on origin memory region

[Philippe Mathieu-Daudé]

Since commit 2cdfcf272d ("memory: assign MemoryRegionOps to all
regions"), all newly created regions are assigned with
unassigned_mem_ops (which might be then overwritten).

When using aliased container regions, and there is no region mapped
at address 0 in the container, the memory_region_dispatch_read()
and memory_region_dispatch_write() calls incorrectly return the
container unassigned_mem_ops, because the alias offset is not used.

Consider the following setup:

    +--------------------+ < - - - - - - - - - - - +
    |     Container      |  mr
    |  (unassigned_mem)  |                         |
    |                    |
    |                    |                         |
    |                    |  alias_offset
    +                    + <- - - - - - +----------+---------+
    | +----------------+ |              |                    |
    | |  MemoryRegion0 | |              |                    |
    | +----------------+ |              |       Alias        |  addr1
    | |  MemoryRegion1 | | <~ ~  ~  ~ ~ |                    | <~~~~~~
    | +----------------+ |              |                    |
    |                    |              +--------------------+
    |                    |
    |                    |
    |                    |
    |                    |
    | +----------------+ |
    | |  MemoryRegionX | |
    | +----------------+ |
    | |  MemoryRegionY | |
    | +----------------+ |
    | |  MemoryRegionZ | |
    | +----------------+ |
    +--------------------+

The memory_region_init_alias() flow is:

  memory_region_init_alias()
  -> memory_region_init()
     -> object_initialize(TYPE_MEMORY_REGION)
        -> memory_region_initfn()
           -> mr->ops = &unassigned_mem_ops;

Later when accessing offset=addr1 via the alias, we expect to hit
MemoryRegion1. The memory_region_dispatch_read() flow is:

  memory_region_dispatch_read(addr1)
  -> memory_region_access_valid(mr)   <- addr1 offset is ignored
     -> mr->ops->valid.accepts()
        -> unassigned_mem_accepts()
        <- false
     <- false
   <- MEMTX_DECODE_ERROR

The caller gets a MEMTX_DECODE_ERROR while the access is OK.

Fix by dispatching aliases recursively, accessing its origin region
after adding the alias offset.

Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
---
v4:
- added ASCII schema
v3:
- reworded, mentioning the "alias to container" case
- use recursive call instead of while(), because easier when debugging
  therefore reset Richard R-b tag.
v2:
- use while()
---
 softmmu/memory.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/softmmu/memory.c b/softmmu/memory.c
index d4493ef9e43..b899ca6a6b7 100644
--- a/softmmu/memory.c
+++ b/softmmu/memory.c
@@ -1442,6 +1442,11 @@ MemTxResult memory_region_dispatch_read(MemoryRegion *mr,
     unsigned size = memop_size(op);
     MemTxResult r;
 
+    if (mr->alias) {
+        return memory_region_dispatch_read(mr->alias,
+                                           mr->alias_offset + addr,
+                                           pval, op, attrs);
+    }
     if (!memory_region_access_valid(mr, addr, size, false, attrs)) {
         *pval = unassigned_mem_read(mr, addr, size);
         return MEMTX_DECODE_ERROR;
@@ -1486,6 +1491,11 @@ MemTxResult memory_region_dispatch_write(MemoryRegion 
*mr,
 {
     unsigned size = memop_size(op);
 
+    if (mr->alias) {
+        return memory_region_dispatch_write(mr->alias,
+                                            mr->alias_offset + addr,
+                                            data, op, attrs);
+    }
     if (!memory_region_access_valid(mr, addr, size, true, attrs)) {
         unassigned_mem_write(mr, addr, data, size);
         return MEMTX_DECODE_ERROR;
-- 
2.26.3