On 23 February 2017 at 15:21, Paolo Bonzini <pbonz...@redhat.com> wrote: > > > On 23/02/2017 15:35, Peter Maydell wrote: >> On 23 February 2017 at 12:53, Paolo Bonzini <pbonz...@redhat.com> wrote: >>> >>> >>> On 23/02/2017 13:26, Peter Maydell wrote: >>>> On 23 February 2017 at 11:43, Paolo Bonzini <pbonz...@redhat.com> wrote: >>>>> On 23/02/2017 12:34, Peter Maydell wrote: >>>>>> We should probably update the doc comment to note that the >>>>>> pointer is to host-endianness memory (and that this is not >>>>>> like normal RAM which is target-endian)... >>>>> >>>>> I wouldn't call it host-endianness memory, and I disagree that normal >>>>> RAM is target-endian---in both cases it's just a bunch of bytes. >>>>> >>>>> However, the access done by the MemoryRegionOps callbacks needs to match >>>>> the endianness declared by the MemoryRegionOps themselves. >>>> >>>> Well, if the guest stores a bunch of integers to the memory, which >>>> way round do you see them when you look at the bunch of bytes? >>> >>> You see them in whatever endianness the guest used. >> >> I'm confused. I said "normal RAM and this ramdevice memory are >> different", and you seem to be saying they're the same. I don't >> think they are (in particular I think with a BE guest on an >> LE host they'll look different). > > No, they look entirely the same. The only difference is that they go > through MemoryRegionOps instead of memcpy.
Then we have a different problem, because the thing this patch is claiming to fix is that the memory the device is backed by (from vfio) is little-endian and we're not accessing it right. RAM of the usual sort is target-endian (by which I mean "when the guest does a write of 32-bits 0x12345678, and you look at the memory byte by byte then the order of bytes is either 0x12 0x34 0x56 0x78 if TARGET_LITTLE_ENDIAN or 0x78 0x56 0x34 0x12 if TARGET_BIG_ENDIAN"). AIUI what we want for this VFIO case is "when the guest does a 32-bit write of 0x12345678 then the bytes are 0x12 0x34 0x56 0x78 regardless of whether TARGET_BIG_ENDIAN or not". thanks -- PMM
