Re: [Qemu-devel] [Qemu-ppc] [PATCH v3 2/4] ppc64-dump: Support dump for little endian ppc64

[Alexander Graf <agraf@suse.de>]

On 05/07/2014 10:54 PM, Tom Musta wrote:
> On 5/7/2014 2:02 PM, Tom Musta wrote:
>
>> It feels like there is an endianness issue here but I have not yet been able
>> to put my finger on it.
> OK ... after more thought and scribbling ... here is what I mean ....
>
> Suppose I have a 64-bit value 0xa0a1a2a3a4a5a6a7 stored in guest memory and it
> gets loaded into a GPR.  If I follow the dump code and view all four combinations
> of guest/host big/little endian, I convince myself that the big endian guest
> code writes the byte sequence 0xa0, 0xa1, 0xa2, ..., 0xa7 into the file.  And
> the little endian guest dumps contain the sequence 0xa7, 0xa6, ..., 0xa0.
>
> This make sense ... the endianness indicated in the dump header and the endianness of
> the dump file layout are consistent, irrespective of the host endianness.
>
> If I take this a step further and consider a 128-bit value 0xa0a1a2a3a4a5a6a7a8a9aaabacadaeaf
> stored in guest memory and look at the AVR structure (via printf or debugger) after doing
> a 128-bit lvx load, I get the following:
>
>    +------+-------+------------------+------------------+---------------------+
>    | Host | Guest | avr.u64[0]       | avr.u64[1]       | file sequence       |
>    +------+-------+------------------+------------------+---------------------+
>    | BE   | BE    | a0a1a2a3a4a5a6a7 | a8a9aaabacadaeaf | a0,...,a7,a8,...,af |
>    | LE   | BE    | a8a9aaabacadaeaf | a0a1a2a3a4a5a6a7 | a8,...,af,a0,...,a7 |
>    | BE   | LE    | a0a1a2a3a4a5a6a7 | a8a9aaabacadaeaf | a7,...,a0,af,...,a8 |
>    | LE   | LE    | a8a9aaabacadaeaf | a0a1a2a3a4a5a6a7 | af,...,a8,a7,...,a0 |
>    +------+-------+------------------+------------------+---------------------+
>
> The last column represents how I think the proposed dump code will write bytes
> to disk.  Notice that if you invert the (64-bit) array elements, then the two
> BE dumps look alike and the two LE dumps look alike.  If you swap array u64
> elements on LE hosts, and also swap on LE guests, then you get a byte sequence
> that looks like a 128-bit integer in all cases.

Since we're dumping an ELF core dump the big question is what gdb wants 
to load :)


Alex