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[Qemu-devel] Re: [PATCH] Always swap endianness in DBDMA
From: |
Alexander Graf |
Subject: |
[Qemu-devel] Re: [PATCH] Always swap endianness in DBDMA |
Date: |
Wed, 23 Dec 2009 11:21:12 +0100 |
On 23.12.2009, at 11:06, Aurelien Jarno wrote:
> 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.
Yeah, I was thinking of a helper function to convert that, so we don't have
#ifdef's cluttered all over the code :-)
Alex
- [Qemu-devel] Re: [PATCH] Always swap endianness in DBDMA, (continued)
- [Qemu-devel] Re: [PATCH] Always swap endianness in DBDMA, Michael S. Tsirkin, 2009/12/22
- Re: [Qemu-devel] Re: [PATCH] Always swap endianness in DBDMA, Paul Brook, 2009/12/23
- Re: [Qemu-devel] Re: [PATCH] Always swap endianness in DBDMA, Laurent Vivier, 2009/12/23
- Re: [Qemu-devel] Re: [PATCH] Always swap endianness in DBDMA, Aurelien Jarno, 2009/12/23
- Re: [Qemu-devel] Re: [PATCH] Always swap endianness in DBDMA, Alexander Graf, 2009/12/24
- Re: [Qemu-devel] Re: [PATCH] Always swap endianness in DBDMA, Laurent Vivier, 2009/12/23
[Qemu-devel] [PATCH] Always swap endianness in DBDMA, Alexander Graf, 2009/12/22