Re: [Qemu-devel] [PATCH] fw_cfg: fix endianness in fw_cfg_data_mem_read() / _write()

[Laszlo Ersek <lersek@redhat.com>]

On 12/31/14 18:58, Laszlo Ersek wrote:
> On 12/31/14 18:44, Peter Maydell wrote:
>> On 31 December 2014 at 17:17, Peter Maydell <peter.maydell@linaro.org> wrote:
>>> Unpatched QEMU + QEMU_EFI.fd.v4 : doesn't boot (doesn't load kernel)
>>> Patched QEMU + QEMU_EFI.fd.v4_noswizzle : boots OK (dracut-initqueue
>>> starts downloading things).
>>
>> Just noticed you asked for the full matrix of tests:
>> patched QEMU + fd.v4 : doesn't boot (doesn't load kernel)
>> unpatched + noswizzle : boots OK
>>
>> That's a bit confusing...
> 
> It's not confusing. When you run unpatched qemu *on a big endian host*,
> that's identical to what the patched code does *on a big endian host*.
> 
> The full matrix actually has 8 elements (3 dimensions with 2 values per
> dimension) -- host endianness, patched qemu vs. unpatched qemu, and
> swizzling vs. non-swizzling firmware.

To elaborate a bit more (I hope I can manage after several glasses of wine):

The central idea is that the firmware should not swizzle directly in the
transport code. It should only swizzle dependent on the individual
integer encodings in the payload for a given key. QEMU's read accessor
should build the host-endian representation of the big-endian
interpretation of the fw_cfg *(sub)string*. This is what the patch does.

The matrix is (guest endianness is invariably LE):

qemu     firmware swizzles       host        boots
patched  (== firmware is buggy)  endianness
-------  ----------------------  ----------  -------------------------
0        0                       BE          yes (your test #4)
0        0                       LE          no (tested right now)

0        1                       BE          no (your test #1)
0        1                       LE          yes (my earliest test)

1        0                       BE          yes (your test #2)
1        0                       LE          yes (my test for this p.)

1        1                       BE          no (your test #3)
1        1                       LE          no (tested right now)

Thanks
Laszlo