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

["Philippe Mathieu-Daudé" <f4bug@amsat.org>]

Peter Xu already reviewed, but Cc'ing Peter Maydell too due to
his last comment on v3:
https://www.mail-archive.com/qemu-devel@nongnu.org/msg800482.html

On 4/18/21 7:57 AM, 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.
> 
> 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;
>