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
