Re: [Qemu-devel] [PATCH v4 1/8] fw_cfg: max access size and region size are the same for MMIO data reg

[Laszlo Ersek]

On 12/16/14 22:47, Paolo Bonzini wrote:
> On 16/12/2014 21:17, Laszlo Ersek wrote:
>>>> I can't imagine how that would happen; fw_cfg_data_mem_read() ignores
>>>> both "addr" and "size", and fw_cfg_read() simply advances the
>>>> "cur_offset" member.
>> Ah okay, I understand your point now; you're probably saying that
>> access_with_adjusted_size() traverses the offsets in the wrong order.
>> ... I don't see how; the only difference in the access() param list is
>> the shift count. (I don't know how it should work by design.)
> 
> I think I have figured it out.
> 
> Guest endianness affects where those bytes are placed, but not the order
> in which they are fetched; and the effects of guest endianness are
> always cleaned up by adjust_endianness, so ultimately they do not matter.
> 
> Think of how you would implement the uint64_t read in fw_cfg:
> 
> File bytes         12 34 56 78 9a bc de f0
> 
> fw_cfg_data_mem_read should read
> 
> size==4 BE host    0x12345678
> size==4 LE host    0x78563412
> size==8 BE host    0x123456789abcdef0
> size==8 LE host    0xf0debc9a78563412
> 
> So the implementation of fw_cfg_data_mem_read must depend on host
> endianness.  Instead, memory.c always fills in bytes in the same order,
> on the assumption that the reads are idempotent.

I see. Thanks!
Laszlo