Re: [Qemu-devel] [PATCH v5 11/15] memory: Single byte swap along the I/O path

[Paolo Bonzini <pbonzini@redhat.com>]

On 26/07/19 08:47, tony.nguyen@bt.com wrote:
> +static bool memory_region_endianness_inverted(MemoryRegion *mr)
>  {
>  #ifdef TARGET_WORDS_BIGENDIAN
>      return mr->ops->endianness == DEVICE_LITTLE_ENDIAN;
> @@ -361,23 +361,27 @@ static bool
> memory_region_wrong_endianness(MemoryRegion *mr)
>  #endif
>  }
>  
> -static void adjust_endianness(MemoryRegion *mr, uint64_t *data,
> unsigned size)
> +static void adjust_endianness(MemoryRegion *mr, uint64_t *data, MemOp op)
>  {
> -    if (memory_region_wrong_endianness(mr)) {
> -        switch (size) {
> -        case 1:
> +    if (memory_region_endianness_inverted(mr)) {
> +        op ^= MO_BSWAP;
> +    }

Here it should not matter: the caller of memory_region_dispatch_read
should includes one of MO_TE/MO_LE/MO_BE in the op (or nothing for host
endianness).  Then memory_region_endianness_inverted can be:

  if (mr->ops->endianness == DEVICE_NATIVE_ENDIAN)
    return (op & MO_BSWAP) != MO_TE;
  else if (mr->ops->endianness == DEVICE_BIG_ENDIAN)
    return (op & MO_BSWAP) != MO_BE;
  else if (mr->ops->endianness == DEVICE_LITTLE_ENDIAN)
    return (op & MO_BSWAP) != MO_LE;

and adjust_endianness does

  if (memory_region_endianness_inverted(mr, op)) {
    switch (op & MO_SIZE) {
      ...
    }
  }

I think the changes should be split in two parts.  Before this patch,
you modify all callers of memory_region_dispatch_* so that they already
pass the right endianness op; however, you leave the existing swap in
place.  So for example in load_helper you'd have in a previous patch

+        /* FIXME: io_readx ignores MO_BSWAP.  */
+        op = SIZE_MEMOP(size) | (big_endian ? MO_BE : MO_LE);
         res = io_readx(env, &env_tlb(env)->d[mmu_idx].iotlb[index],
-                       mmu_idx, addr, retaddr, access_type,
SIZE_MEMOP(size));
+                       mmu_idx, addr, retaddr, access_type, op);
         return handle_bswap(res, size, big_endian);

Then, in this patch, you remove the handle_bswap call as well as the
FIXME comment.

Paolo