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

From: Mark Cave-Ayland
Subject: Re: [PATCH v3] memory: Directly dispatch alias accesses on origin memory region
Date: Mon, 19 Apr 2021 21:13:07 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.9.0

On 17/04/2021 15:02, Philippe Mathieu-Daudé wrote:

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;

Whilst working on my q800 patches I realised that this was a similar problem to the one I had with my macio.alias implementation at [1]: except that in my case the unassigned_mem_ops mr->ops->valid.accepts() function was being invoked on a ROM memory region instead of an alias. I think this is exactly the same issue that you are attempting to fix with your related patch at https://lists.gnu.org/archive/html/qemu-devel/2021-04/msg03190.html ("memory: Initialize MemoryRegionOps for RAM memory regions").

I eventually realised that I needed functions that could dispatch reads/writes to both IO memory regions and ROM memory regions, and that functionality is covered by the address_space_*() access functions. Using the address_space_*() functions I was then able to come up with the working implementation at [2] that handles accesses to both IO memory regions and ROM memory regions correctly.

The reason I initially used the memory_region_dispatch_read()/memory_region_dispatch_write() functions was because I could see that was how the virtio devices dispatched accesses through the proxy. However I'm wondering now if this API can only be used for terminating IO memory regions, and so the alias_offset that you're applying above should actually be applied elsewhere instead.





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