[Qemu-devel] Re: [PATCH] Always swap endianness in DBDMA

[Alexander Graf]

On 23.12.2009, at 07:12, Aurelien Jarno wrote:

> On Tue, Dec 22, 2009 at 02:45:17PM +0100, Alexander Graf wrote:
>> When we get an MMIO request, we always get variables in host endianness. The
>> only time we need to actually reverse byte order is when we read bytes from
>> guest memory.
>> 
>> Apparently the DBDMA implementation is different there. A lot of the logic
>> in there depends on values being big endian. Now, qemu does all the 
>> conversion
>> in the MMIO handlers for us already though, so it turns out that we're in
>> the same byte order from a C point of view, but cpu_to_be32 and be32_to_cpu
>> end up being nops.
>> 
>> This makes the code work differently on x86 (little endian) than on ppc (big
>> endian). On x86 it works, on ppc it doesn't.
>> 
>> This patch (while being seriously hacky and ugly) makes dbdma emulation work
>> on ppc hosts. I'll leave the real fixing to someone else.
> 
> I have to say I found it too hacky to be included in QEMU. I would
> prefer if someone can provide a real fix.

Looking at hw/pci_host_template.h I'm actually more confident now that this is 
actually the correct solution. We don't want big endian or little endian, we 
want reversed endian variables.

Alex