[Qemu-devel] Re: [PATCH 00/15] [RFC] MMIO endianness cleanup

[Paul Brook <paul@codesourcery.com>]

> The way mmio endianness is currently implemented is horrifying.

Agreed.

> #ifdef TARGET_WORDS_BIGENDIAN
>     val = bswap32(val);
> #endif
> 
> With the move to get device code only compiled once, this has
> become harder and harder to justify though, since we don't know
> the target endianness during compile time.

Not just that, it's wrong to start with.  I've used machines with both native 
and cross-endian 16550 based UARTs.
 
> So my solution to the issue is to make every device define if
> it's a little, big or native (target) endianness device. This
> basically tells the layers below what endianness the device
> expects mmio to occur in. Little endian devices on little endian
> hosts don't swap. On big endian hosts they do. Same the other
> way around.
> 
> The only reason I added "native" endianness is that we have some
> PV devices like the fw_cfg that expect qemu's broken behavior.
> These devices are the minority though. In the long run I'd expect
> to see most code be committed with either of the two endianness
> choices.

I'd prefer to avoid this, or at least document it as a temporary hack that 
should be removed.  If a device can exist in either endian, then we really 
want to push this decision down to the board-level code.

One of the reasons I haven't bothered fixing this yet is that this feels like 
something that should be a device/bus property.  e.g. PCI devices/busses are 
always little-endian[1], as as mentioned above some devices come in both 
flavors.  I guess we can go with your approach for now, and make sure we fix 
this properly when we introduce bus-specific registration functions.

Paul

[1] Ignoring magical byteswapping cpu-pci bridges, but they're broken by 
design, and thankfully quite rare.