Re: [Qemu-devel] [PATCH 2/3] s390: implement pci instructions

[Paolo Bonzini <pbonzini@redhat.com>]

On 12/11/2014 10:22, Alexander Graf wrote:
>>>> Absolutely lets make an example for qemu running on BE and LE
>>>>
>>>> byte order    config space backing   pci_default_read_config   pcilg (with cpu_to_le)
>>>> BE            0x78563412             0x12345678                0x78563412
>>>> LE            0x78563412             0x78563412                0x78563412
>>>
>>> No, pci_default_read_config() always returns 0x12345678 because it
>>> returns a register, not memory.
>>>
>>
>> You mean implementation of pci_default_read_config is broken?
>> If it should return a register it should not do "return le32_to_cpu(val);"
> 
> It has to, to convert from memory (after memcpy) to an actual register
> value. Look at the value list in Paolo's email - I really have no idea
> how to explain it any better.

pci_default_read_config is reading from a *device* register, and has
absolutely zero knowledge of the host CPU endianness.

Another way to explain that the result of pci_default_read_config is
independent of the host endianness, is that the function is basically
doing this:

switch (len) {
    case 1: return d->config[address];
    case 2: return ldw_le_p(&d->config[address)]);
    case 4: return ldl_le_p(&d->config[address)]);
    default: abort();
}

So if you want to make the outcome big endian, you have to swap
unconditionally.

Paolo