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[PATCH v3] memory: Directly dispatch alias accesses on origin memory reg

From: Philippe Mathieu-Daudé
Subject: [PATCH v3] memory: Directly dispatch alias accesses on origin memory region
Date: Sat, 17 Apr 2021 16:02:06 +0200

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.

The memory_region_init_alias() flow is:

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

Later when accessing the alias, the memory_region_dispatch_read()
flow is:

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

The caller gets a MEMTX_DECODE_ERROR while the access is OK.

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

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

diff --git a/softmmu/memory.c b/softmmu/memory.c
index d4493ef9e43..23bdbfac079 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,
+                                           addr + mr->alias_offset,
+                                           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 
     unsigned size = memop_size(op);
+    if (mr->alias) {
+        return memory_region_dispatch_write(mr->alias,
+                                            addr + mr->alias_offset,
+                                            data, op, attrs);
+    }
     if (!memory_region_access_valid(mr, addr, size, true, attrs)) {
         unassigned_mem_write(mr, addr, data, size);
         return MEMTX_DECODE_ERROR;

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