Re: [PATCH v2] memory: Directly dispatch alias accesses on origin memory region

[Paolo Bonzini <pbonzini@redhat.com>]

On 22/08/20 18:14, Philippe Mathieu-Daudé wrote:
> There is an issue when accessing an alias memory region via the
> memory_region_dispatch_read() / memory_region_dispatch_write()
> calls.
> 
> Since commit 2cdfcf272d ("memory: assign MemoryRegionOps to all
> regions") we assign &unassigned_mem_ops to all MemoryRegions.
> 
> 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;
> 
> When accessing the alias using memory_region_dispatch_read(),
> flow is:
> 
>   memory_region_dispatch_read()
>   -> memory_region_access_valid(mr)
>      -> 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 directly dispatching aliases accesses to its origin region.
> 
> Fixes: 2cdfcf272d ("memory: assign MemoryRegionOps to all regions")
> Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>

It's still not clear from the commit message what is the case where you
get this.  I can integrate it in the commit message myself, but can you
explain it and/or link to a bug report?

Paolo