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

[Aurelien Jarno]

On Wed, Dec 23, 2009 at 10:29:53AM +0100, Alexander Graf wrote:
> 
> 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.
> 

I guess it's the same problem, that is the bswap is there to emulate the
fact that the bus is connected backward. Then I guess that the bswap 
should also depends on TARGET_WORDS_BIGENDIAN, even if this file is 
currently only compiled for big endian targets. Also a comment might be 
a good idea.

Then I guess the patch is acceptable into QEMU.

-- 
Aurelien Jarno                          GPG: 1024D/F1BCDB73
address@hidden                 http://www.aurel32.net